Are you looking to buy just a graphics card (as the title suggests), or an entirely new system, as the message suggests?
For 1280x1024, a HD6770 would be enough; they're around $110 now, and can be powered by a reasonable 430W PSU (or an excellent 380W-400W model).
If you'd like to anticipate playing on ultra settings at 1920x1080, I can play Skyrim at that resolution maxed with a GTX560Ti. That GPU needs a good 500W PSU. If you were to choose an AMD equivalent, it would be somewhere between a HD6870 and a HD6950 (depending on the game).