I actually bought a used one from someone on the FS forums here (syndicate). It was originally from an XPS system he upgraded, but I believe most 7900GS's should work as they are not usually that long. I think the 7900GS is a better deal than a 8600GT since it's still a while before DX10 games are going to be popular and I think I remember seeing something like the 8600 performs like a 7600 with DX10 support so I think a 7900GS is more powerful for today's games.
Before my upgrade to a 7900gs I had an ATI x800 Pro in there which worked very well also. In fact, I don't really notice any difference between the two for the games I play other than the 7900gs fan is much louder. I only upgraded because the fan on my x800 Pro was making a clicking noise like one of the wires was hitting the fan. If you can get a good deal on an x800 Pro, I'd recommend that card also.
Also, the games I've played on the 7900gs are C&C3, Supreme Commander, Company of Heroes, Call of Duty 2, and Heroes of Might and Magic 5. All of them are set on the highest resolutions my monitor can handle (1680x1050) and on high graphics settings. All of the games run very smooth.