Current GPU: EVGA Geforce GTX 650 2GB
At the current time you best bet is either the gtx 780 - $400
or the r9 280x - $329 give or take.
The 780 is better but also significantly more, the 280x is a good card and once some of the driver issues get sorted out it should be perfect for you, also, what's your cpu? I just want to make sure you won't be bottlenecking yourself.