The 7900gt 512MB would generally be a little faster than the 8600, but there may be some driver issues using it on a newer motherboard, (discontinued support and all). Of course, it's only a DX9 card. Of the DX10 cards the 8600gt 512 would be the fastest. The 2600XT would be limited by the small 256MB and the 4550/4350 would be crap for any recent games.
If you're buying something used off of eBay then just go with a Radeon 3850 512MB as they should be rather cheap and are much faster than the cards you listed.
If you are going to be building a new system in two months don't bother wasting money on a temporary card. Just buy the video card for it a few months early.
What resolution is your monitor? Or what it will be on your new one if you are upgrading it to go with the new system.
i am waiting for Fermi what about eForce 8800GS Superclocked 384MB i found one for 30us on ebay people are selling the hd 3850 for 100 or more i mean really, also an 2900xt for 499us is the 2900xt some type of super card thats way to much, i game at 1280-960