Well their is alot more to what makes a video card good, besides more memory. More memory is great, but memory of course is just space for data. Sometimes your not really using all that memory, so its not making that much of a speed increase.
Your really going to want 256MB and 256Bit videocards.
But most important, you want the memory to be GDDR3, this is the best. Now the 9800 pro, is a 256bit video card, but dont have GDDR3. The 6600 GT is not 256bit, but has GDDR3. That has alot to do with why the 6600 is better.
When your looking at a video card, this is what you will see.
Chipset/Core Speed : around 350mhz, anything 400+ is great
Pixel Pipeline depends: if its PCI Express/AGP ect
Memory/Effective Speed: 900mhz+ anything over 1ghz is ideal
Now i just cant throw a number at you and say, this its average. Because it all depends on the type of card. Rather its PCI, or AGP. Or its a 64 Bit video card or a 256bit video card, ect. But in general, the speeds i listed are pretty good to follow.
Finished With TF.