it might be beneficial to take a look at craigslist or ebay to see whats lurking around that price range, you might find a nice upgrade, who knows?
thx v4800(based off the hd56xx cards) is by far the best card in the bunch, close enough to equivalent to your 8800gts, everything else is much slower, especially the nvidia.
the cards should all support windows 7, the drivers base what cards can or cant support it anyways, and pcie 16x is all thats required, even if its the older 1.0 spec its fast enough.
plus the gig of ram depending on the program is likely a nice thing to have.
Athlon II X2 245, Deepcool Gamerstorm hsf(w/ Deepcool SF120 & Cougar CF-V12HP fans), 4gb ddr2-800, GTX 460 2gb, Auzentech X-Mystique, Gigabyte M68M S2P, Win 7 Ult x64 -desktop
HP Elitebook 8530p - Intel P8700, 3gb ddr2, 80gb intel x25-m, ati hd3650