I assume building your own isn't a real option so you're a bit limited...
If you're willing to go up to 850USD, get this one: http://www.newegg.com/Product/Produc...82E16883103854
It's the most ideal one I could find up to about 900USD.
Here's one for 750USD: http://www.newegg.com/Product/Produc...82E16883220444 (found in the clearance section so don't wait too long!)
CPU's a bit slower but that pretty much only will affect you in applications optimized for multithreaded usage. (read: currently N/A for gaming)
If you plan on using this PC for a long time, it might be worth considering the first I linked due to a multithreaded CPU and the i7 is generally a tad more powerful overall. (My old Intel Q9550 lasted 4 years with fairly good performance before I decided to upgrade)
You also might want to wait for recommendations of the other regulars in this subsection. They could recommend a better PC.