Heya,
ATI - The HD4850 is in the $100 price range.
nVidia - The 9800GT is in the $100 price range.
-- Though you can find the GTX260 and HD4870 in the $100 price range if you look for open-box and recertified deals on the egg or other places.
Try and get the 1Gb memory versions, not the 512Mb.
As for PSU, all you need is a single 6pin PCIe connection. Any decent PSU can handle these cards that are rated for about 500 watts for sure. Corsair, Seasonic, OCZ, Raidmax, Apevia, even the Rosewell (seriously) PSUs are all perfectly fine. I think there's a deal right now on the egg:
650W Corsair for $88.99 if you use promocode EMCMNNM55
Link (also has MIR of $20).
Very best,