PC Upgrade videocard

Hey guys got this old pc i wanna do some upgrades on, its used mostly for minor design, 3d and rendering.

Here are the spec

Athlon 3500+ Venice
RD580-sb450 (939) ati xpress 3200 chipset MB
2.0 GB OCZ Ram
7900 GT/GTO bfg videocard.

anyways, looking to make this a faster computer, without changing the motherboard, starting with the video card and CPU. Any recommendation and any good deals? new or used. Havent been paying much attention to pc hardware, but so there pci-e 2.0 now, so newer cards might not work correctly with my motherboard which is PCI-E x 16

Ive seen some video cards for sale on local forums, NEW BFG 9800 GTX 512 $120

Also someone is selling a 880gtx for 80 bucks which seems to be too cheap, not to sure..

http://www.redflagdeals.com/forums/evga-8800gtx-w-blast...

my one concern is that i dont want to buy something that is too fast, and my pc cant even utilize its full potential.

Thanks