5770 Vapor-X is the fastest with most cooling potential for overclocking. If you have no intentions of overclocking the card, go with the 5770 that WhyFly suggested.
OK, I have the GTS250 and HD5770. I must say, the GTS250 is an amazing card for the price. You get DX10, PhysX, CUDA, and great framerates. It also runs pretty cool. The HD5770 is certainly the superior card though (IN TERMS OF POWER, it does not have PhysX which I sort of miss (but I use my GTS250 as a PhysX card so it's all good )), and has DX11 to boot. So, unless PhysX is important to you (I absolutely love Mirrors Edge, so it is to me) then go for the 5770.