Well to find out if you have AGP or PCI or PCI-e slots, you need to run an everst scan. http://www.majorgeeks.com/download4181.html
PCI is the oldest and worst, AGP is good, but it will die this year, you can get a 7800 GS, then PCI-e is the newest and fastest .. soon PCI-e2 will come out tho.
Five looks like the best .. but those are all very poor cards.
If you only have PCI .. get this one .. *the the best PCI card .. still horrible, but if you can't upgrade your mobo*
http://www.newegg.com/Product/Product.asp?Item=N82E16814143032
Now if you have AGP you can get the 7800 GT which is a great card
http://www.newegg.com/Product/Product.asp?Item=N82E16814130274
Now if you have PCI-e you can get a nice 7900 GTX
http://www.newegg.com/Product/Product.asp?Item=N82E16814130280
or another good PCI-e card would be the x1900xtx
http://www.newegg.com/Product/Product.asp?Item=N82E16814121553
When shopping for a video card, if you are a gamer you will want at least 256mb of ram, but 512 is getting quit popular. Having 20 pixel pipes is preety good too. HAving SM 2 and 3 and usually needed for any new games.