I was wondering why a custom built PC is nearing the price of a new MAC, then I seen the $800 monitor.
Like Rathji said, a smaller SSD drive would do wonders for your total price.
The GTX 570 is the second best from Nvidia from what I can see, I don't see a need to go higher. I have a GTX 460 and I can still run any game that is released today in the highest graphic settings.
http://www.videocardbenchmark.net/high_end_gpus.html