making sure you buy the right form factore... agp or pci-express. In your case agp. at least 128 megs ram of onboard video memory (256 prefered). and always buy the fastest you can afford.
you would have to look up your system specs. depends on how old your system is. If you only have 4x agp then I wouldn't go over a 6600. if its agp then I would look for a 6800. it may be slightly more than you intended to spend.
we use http://www.alexnolan...are/sysspec.htm
as a handy tool to look up your system specs.