I'd try staying under 1 grand.
Maybe in the 700 range, I guess.
Not going to get a good stock PC that can run max shaders for that little.
You could try doing
this if you want some pretty awesome performance for the price. With the leftover cash, you can get a good keyboard/mouse and monitor, too.
If you're not up for building a PC, (It's pretty easy) you could try buying a $700 PC and slapping in a GTX 660-range card w/ some extra cooling. (And power supply, if it's nessesary) If you do that, though, you should make sure that the mobo and such are compatible. I understand that you wanted a stock computer, but I can't stress enough that the performance difference between a PC like the one I linked earlier and a stock one will be huge.
Either way, I would suggest consulting the Tom's Hardware forum.