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Video controllers translate byte values deposited in their video memory by your GUI (usually an X server under Linux) into an analog RGB signal which drives your monitor. The simplest kinds treat their video memory as one big frame buffer, requiring the CPU to do all dot-painting. More sophisticated ``accelerated'' cards offer operations such as BitBlt so your X server can hack the video memory algorithmically. These days almost all cards even at the low end actually have some acceleration features.
Cards are rated by the maximum number of analog signal changes they can produce per second (video bandwidth). Video bandwidth can be used to buy varying combinations of screen resolution and refresh speed, depending on your monitor's capabilities.
Another important variable of video cards is the size of their on-board video RAM. Increased memory lets you run more colors at higher resolutions. For instance, a 1MB card usually will only allow 256 colors at 1024x768 while a 2MB card usually allows at least 16-bit color (a palette of about 65,000 colors). You'll need 4MB of video memory to use 24-bit or ``true'' color (16 million colors) at 1024x768.
The card's video RAM size has no effect on its speed. What does affect speed is the type of memory on board. VRAM (Video Random Access Memory) is fast but more expensive; it features a dual-ported design allowing two devices (the CRT controller and the CPU) to access the memory at the same time. DRAM (Dynamic Random Access Memory) is is similar to the RAM used in main memories. It is cheaper, more common, and slower (because the CRT controller and the CPU must take turns accessing the video buffer).
A quick review of monitor standards:
Horizontal Vertical
Name Resolution Colors Frequency Frequency Notes
------------------------------------------------------------------------
MDA 720x350 18.43 KHz 50 Hz Obsolete
CGA 640x200 2 15.85 KHz 60 Hz Obsolete
320x200 4
EGA 640x350 16 21.80 KHz 60 Hz Obsolete
VGA 640x480 16 31.50 KHz 60 Hz
320x200 256
VESA VGA 640x480 16 38.86 KHz 72 Hz
320x200 256
VESA SVGA 800x600 16 48.01 KHz 72 Hz
640x480 256
8514/A 1024x768 16 35.20 KHz 43.5 Hz Obsolete
XGA 1024x768 256 ?? ?? Obsolete
VESA 1024x768 1024x768 256 56.48 KHz 70 Hz
The Horizontal and Vertical Frequency columns refer to the monitor scan frequencies.
The vertical frequency is the upper limit of the monitor's flicker rate; 60Hz is minimal for ergonomic comfort, 72Hz is VESA-recommended, and 80Hz is cutting-edge. At resolutions above VGA, horizontal scans take long enough that the monitor may never reach anywhere near the vertical-frequency maximum; how close it gets is a function of the horizontal-scan frequency (higher is better).
For more information on how to avoid the evil screen flicker, see "The Hitchhiker's Guide to X386 Video Timing (or, Tweaking Your Monitor Modes for Fun and Profit)", a tutorial written by your humble editor and included with the XFree86 distribution; surf to it at https://tuxedo.org/~esr/faqs/video-modes
It's still barely possible to find MDA video boards and monitors out there, but prices for SVGA have collapsed so totally that it's not worth bothering.
XGA is an IBM-proprietary included for completeness, but is vanishingly rare in the clone market. 8514/A is another IBM standard supported by a few graphics accelerator cards. It is interlaced, and thus has a tendency to flicker. The VESA 1024x768 standard makes XGA and 8514/A obsolete,
SVGA or `Super VGA' strictly refers only to 800x600 resolution, but is widely used for 1024x768 and even 1280x1024 resolutions. Standards above 1024x768 are weak and somewhat confused.
These days, most vendors bundle a 14" monitor and super-VGA card with 1024x768 resolution in with their systems. Details to watch are the amount of RAM included (which will affect how much of that maximum resolution and how many colors you actually get), and whether the memory is dual-ported VRAM (slightly more expensive but much faster).
You should check ahead of time which Super VGA chipset the vendor normally ships. Though DOS/Windows doesn't really care, the Unix software that uses it (most notably X servers) will definitely notice the difference. Many commercial implementations of X servers for Intel Unix don't know how to use the SVGA modes of the cheapie Oak and Trident SVGA chipsets, which are the ones most often bundled with systems (XFree86 handles them OK, though). The cards based on the Tseng 4000 chipsets are often bundled or available as as an extra-cost option (usually less than $50 above Oak or Trident), perform better, and are supported by the most implementations of Intel Unix-based X servers. ATI VGAWonder chipset is well-supported and often bundled with multimedia or graphics systems.
Things to check for on the monitor spec sheet:
If you can, buy your monitor from someplace that will let you see the same monitor (the very unit you will walk out the door with, not a different or `demo' unit of the same model) that will be on your system. There's a lot of quality variation (even in "premium" monitor brands) even among monitors of the same make and model.
Another good reason to see before you buy, and carry it home yourself, is that a lot of monitors are vulnerable to bumps. The yoke can get twisted, producing a disconcerting tilt in the screen image.
The Caveat Emptor guide has a good section on evaluating monitor specifications. And there's a database of monitor specs at The Big Old Monitor List.
This page tells you how I did it. Specific specs and pricing information will date quickly, but the method should still be good years from now.
My existing monitor wasn't bad -- a 17-inch Swan 617 that I could drive at a bit above 1024x768. Still, I yearned for more real estate -- especially vertical real estate, so I could view full PostScript pages using a legible font.
This brings us to our first prescription: be clear about what you want. It's easy, and very expensive, to buy more monitor than you'll really use.
I knew I wanted something in the 19-to-21-inch range, with 1280x1024 or higher resolution. I knew this would probably cost me about $2000, and could afford it. I knew I didn't need one of the monster projection monitors further upmarket, with screen sizes 24" and up. These will typically cost you $4K or so and are too big for desktop use anyway.
I also knew I didn't need one of the special true-color monitors designed for photo composition, making print separations, and so forth. These creatures (always Trinitrons) have better, denser color than conventional tubes but at a hefty price premium (and usually at some cost in available resolution). If all you're going to do most of the time is 16 or 256-color X screens, you don't need this capability.
Here's how to read the specs:
The R field is screen resolution -- horizontal pixels by vertical. If there's a third number it's a claimed refresh frequency in Hz for the given resolution.
The A field is screen size -- nominal size followed by viewable area.
The D field is dot pitch (the effective size of a pixel).
The H field is a horizontal sync frequency range.
The V field is a vertical sync frequency range.
The B field is bandwidth.
The final figure is the manufacturer's suggested retail.
Web: http://www.magnavox.com
Web: http://www.mela-itg.com
Web: http://www.nec.com
Web: http://www.nokia.com
Web: http://www.sony.com
Web: http://www.viewsonic.com
Nor was the choice between the two very hard. ViewSonic's 21PS is $600 less expensive than Nokia's 445X for very similar performance. And, other things being equal, I'd rather buy a monitor from a specialist monitor manufacturer than a general consumer electronics outfit best known for its cellular phones.
So I determined to order a ViewSonic 21PS.
This left me with a second problem. My ATI Mach 32 can't drive a monitor at higher than 1280x1024 resolution and 94MHz bandwidth. So it wouldn't be able to drive the 21PS at 1600x1200. I wound up buying a Mach 64.
The combination worked wonderfully (two years later I discovered that VA Linux Systems buys the same monitor for its high-end systems). The only problem I have with it is that monitor is way bright even dialed down to its dimmest setting. You'll need a strong light in the room where you install it. Also, be aware that the only really convenient way to move one of these monster monitors is with a forklift!