I have a nMP (6-core with D700's) and have tried it both natively and in bootcamp, and I can honestly say my ageing i7 3770K with a GTX 980 Ti kicks it's ass as a gaming platform at 1440p. A single D700 isn't that fast, and Cross Fire is a bit it and miss in terms of performance - it jerks a like a 2$ whore in Skyrim for example. The nMP does make an excellent photo editing platform and VM lab though.
I used to dream of gaming on the Mac, but it's just not worthwhile. The performance is crap compared to a PC half the price and hardly any new titles get released on Mac. Sure have a Mac as your every day computing platform, but for games you'd be better off getting a slightly lower spec Mac and spending the difference on a console, the performance would be much better and all the latest titles would be available and work in full HD.