Having recently gone through a nightmare experience with installing an ATI Radeon 9600SE, my advice would be to do as much research as you can and to know your system hardware well before you buy. You need to know the following at minimum:
The manufacturer and model of your motherboard.
Does the motherboard have an onboard graphics controller?
If it has an onboard graphics controller, what type, make, and manufacturer?
What chipset is being used? (Intel/Non-Intel, manufacturer, model)
Do you have an available AGP slot?
What is the date and version of your AGP drivers?
You may have to do some digging to find those answers, including visiting the websites of the various manufacturers. After you find all the information, then do some research at the websites for the graphics cards you are considering buying and look through their support and FAQ areas to see if there are any known issues regarding compatibility with your hardware. If there are known issues, do they have a fix posted?
As I said, I had a nightmare experience with my card and ended up spending a lot of time and some money I need not have spent if I had done more research up front. Having a bunch of people tell you how great a card is, even if they are experts or tech reviews from a well known site won't do you a lot of good if the card won't work with your hardware. I found out the hard way. Hopefully someone can benefit from my mistake. Good luck.