The first thing you need to establish is has your mother board got an AGP slot? If so what rate is it ie 2x 4x or 8x.
After you've found that out the first thing to ask is whats your budget and what do you want to use the card for?
There are a number of graphics cards out there ranging in low end budget price ones starting at about £40 upto spending hundreds of pounds on one.
I have a geforce FX5200 with 128mb of RAM onboard. Its does the job but isn't upto playing the latest games out price on dabs.com is around £40. I've had frame rates of 42 frames per second in unreal tournament 2003.