I would advise building a desktop computer yourself: that way you can more easily upgrade your hardware over time and pack more into in. Mine is a GTX 560, 8GB RAM, AMD Quad Core, Windows 7 64-bit that cost about £550 ($900) to put together and can run just about anything at high/ultra. If price isn't a matter, I suggest you look up some of the more powerful GTX 6xx's, like the 680/90's; although you'd have to have quite a lot for one of those.
It's a personal choice, but I would recommend you build a desktop yourself rather than get a pre-made PC; they often overprice them so it's cheaper to get individual components.