depends on what you think is best
560Tis start cheap around $200 and the price starts to rise as faster over clocks, different coolers, or display ports are added.
the price raises close to what the 448 core 560Tis cost(which none should beat). They are not true 560Tis, they are really defective 570s GPUs.
They start life with the intention of being a 570 but a bank of the 480 CUDA units are bad leaving 448 so they are made into 560Ti 448 cores. This isn't a bad thing cause you get a slightly slower 570 at a raised 560Ti price, or a really fast 560Ti at less than a 570 price, depending on how you look at it..and if you decide you could run 3-way SLI with the 448 cores(currently the cheapest way to do that)
even though its over your price range, the baddest true 560Ti is the EVGA 560TiWin, its 2 true 384 core 560Tis on a single card aproching the performance of a 580 for less money. the advantage it offeres is it is a 560Ti SLI setup that doesnt need an SLI motherboard cause it works in one slot, and if you do have an SLI motherboard, with 2 installed, quad SLI can be ran( cheapest way to do that). if you dont want ever want quad SLI and only run one 560TiWin, 2 cheaper 560Tis still cost less than the 560TiWin, if you have the board to support it.
Thats what I did last month, a cheap reference(good warranty) 560Ti went in my machine to replace my aging 7800, so i could play newer games, than I bought another one to offload the PhysX to. It gets the frame rate up on the first one a bit, with the intintion of running them in SLI when I get my new motherboard.