If you're worried you'll buy a card, and your PSU can't handle it, then look at a 5670. Basically the same as a 4770, but no external power connectors are required, and gives a benefit over all the other cards listed, DX11. While performance for DX11 won't be the greatest on the 5670 by any means, it'll keep you someone up-to-date compared to the other DX10 cards.
I do like Clown's suggestion though. I had a similar 9800GT (Newegg.com - PNY XLR8 VCG98GTEE1XEB GeForce 9800 GT 1GB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
) and could max out a lot of games with it. I maxed out CoD4 (all highest settings) and could get 250FPS+ on 1440x900, something I have yet to be able to do with my 4870. I ran Crysis at high settings at 32FPS average.