The number of pixel pipelines is one
of the most important things that affects the speed of a video card. What you need to look for is core speed (in Hz) * the number of pipelines. The higher that value is, generally the better the card; that'll give you the number of shader operations per second. And shader operations are becoming more and more used in modern games, so a high value here is a really good thing. One other thing: Memory-bit interface/8 (<that gives you bytes; 8 bits = 1 byte) * the speed of the effective memory (in MHz) / 1000. That'll give you the card's bandwidth, which you need a lot of if you plan on doing eye-candy things such as Anti-aliasing or anisotropic filtering.
Those are probably the two most important technical factors in a graphics card. Other things you may need to consider: GDDR4>GDDR3>DDR2>DDR...Aim for GDDR3 or higher.
Pixel Shader VERSION: You want 3.0, unless anything higher comes out
Amount of memory: 256MB is generally a good amount; more gives you better results in AA and AF. The 128MB 6600GT is still a decent card though regardless, because its speeds are high with a fair amount of pipelines.
Hope this helps, with any luck you'll understand what I'm saying
. (No, I'm not insulting you, I'm insulting myself
.) If you want, I may be able to explain it better in a PM or something.
If any information here is incorrect, someone please correct me. I'm trying to learn all I can about video cards, as I used to go primarily by which number is higher, then quickly learned that that was wrong.