At your resolution you most assuredly do not need to spend even $200 on a video card!! Now, you could use some of that $250 budget to buy a GPU & a new PSU if you want though. Can you give us any specific details on your power supply?
I'd probably recommend you purchase a PSU along with the video card to be safe though.
The GTX 460 1GB & ATI 6850 will both be plenty for your resolution.
What is the CPU? It would be pointless pairing a fast card with a slow processor, particularly at such a low resolution.
Good PSU from Jerrence, quality at a sensible price although for $10 more you can get this flashier modular from OCZ