ATI are more powerful than Nvidia at the moment due to DDR5 use rather than DDR3 on the cards.
Be aware there isnt a huge difference in performance from 8800 to 9900 and in my opinion it isnt worth the cost.
Thing to ask is "what can i play on a newer card i cant on this one",
1. are you running DX10.1 cus youre using vista? if not then why bother with 10.2 cards.
2. new cards require 6pin power supplies on them not the standard 4pin.
3. PCIE-2 is on most of the new cards, does your mobo support v2?
4. Will the rest of the system bottle-neck the new graphics card? Upping the processor to a faster one can imrpove fps, or getting a better mobo can also improve performance.
5. 32bit or 64 bit OS?
Put it like this: way back i had a p2 233 playing TFC with a matrox 1/2meg card and getting 15fps.
I bought a new graphics which was 16 meg and much faster but only got 15-20 fps, why?
My motherboard couldnt do 8x agp and my proc was crap and i needed more memory.
New proc upped fps to about 40, more memory added another 10, new mobo with 8x agp upped it to 70.
Dont buy unless you are sure you need to, prices drop every day and so if you are fine where you are now then dont bother to upgrade unless new games you cant play come out and then buy the graphics as late as possible so the prices can come down.