i would suggest GTS250 because ATI cards will not support nvidia Physx....but if you want to buy a future proof graphic card then buy HD5750 or HD5770, it supports directx11....HD4870 is also a best choice by the way
HD 4870 will beat both those cards in gaming. The only drawback w/ all 3 cards is the 512mb memory. If you can find a deal on a 1gb version that will keep you going for another year or so; which you then can get a dx11 card from either manufacterer cheaper than they are now.
I use a 4870 512mb @ 1920x1080. While I regret not being able to afford the 1gb model at the time (they were all over $300CAD last march, and the 512mb i bought was on a one-day sale for 215, was a no brainer at the time) I have not had a crippled gaming experience. If you can't find a 1gb for a cheap enough price, a 512mb for 109 is still a killer deal, and the best of the 3 you have listed.
that is othe only drawback, but don't underestimate the power of the 5750.. It basically performs on the same level as the GTS 250 and if overclocked a bit it will match the 4870's performance in some games....