yes sir, u need to first figure out exactly what kind of slot you are working with AGP, PCI, PCIe...secondly do not consider an ATI board. Traditionally the ATI cards/drivers have not faired too well with OFF. Mileage may very but take a look at the FAQ under the graphics section. I have an ATI 4670 IceT 1 gig.
right so as Uncleal says, after the card consider the PSU, don't want a lame PSU with high demand on a new flashy card, could end up inducing system instability.
and as Tamper says, don't want to be the bearer of bad news either but buying a card, then a PSU, then considering RAM...see the downward slope?
Once you figure out what type of slot let us know.
JJ