The common answer on the forums is the correct one. You cant go wrong with a GTX 670, though it is about 100 dollars over your budget. Id say save another 100 bucks to get it, or wait 2 or so more months for Nvidia to release the 660 and 660 ti models. Could be sooner, but it will be a month at LEAST from now.
For a top end card right now if you cant wait, in your price range would be a GTX 570 or a Radeon 7850. Though the 7850 has more memory and slightly better specs, the GTX 570 beats it in FPS in just about everything by a good 5-10 fps. Thought it costs 40 dollars more than the 7850. http://www.techspot.com/review/504-amd-radeon-7870-7850...
If I was in your position Id figure out a way to nab am extra 100 bucks and get a GTX 670 and OC it to 680 level. Best option by far.
Exactly like unskol said.........1366x768 is a very undemanding resolution for most modern cards.
If you plan on staying at this resolution for some time then any card will do in the following page at tom's latest 'best gfx cards for the money':
...and you'll save a bit of money from the top end of your budget. Of course, if you plan on getting a higher res tv/monitor then simply chose a card from the same link above. Do this and you'll not go wrong
Remember to overclock your 2500k (the only reason to get the k version) - e.g. cpu intensive games - particularly BF3 multiplayer.