Hi-
Welcome to Discussions!
You don't say what your software needs are, but to make an upgrade feel like an upgrade, a Radeon X800XT, Geforce 6800 GT or Ultra, or a Geforce 7800 GS would be worthwhile.
The Radeon X800XT and Geforce 6800 Ultra were available from Apple, so used OEM cards may be found on eBay/Apple Components.
Those 2 cards as well as the 6800 GT and 7800 GS that I mentioned, are also available on eBay as flashed (WinPC card converted to Mac ROM) cards, and will provide a more economical purchase.
The flashed cards, with the exception of the X800, run exactly as a Mac edition will.
As it is, any card will cost you from $150-$300.