Today, for games a high-clocked quad-core i5 is better than i7 because i7 has Hyper Threading which hinders gaming performance, and also makes games stutter. Also i5 is cheaper.

Building your own is way easier than you think, more rewarding as well. Doing it yourself you'll definitely get more bang for your buck and know that your not getting generic parts. As stated above, I'd go with a larger power supply for sure. And 16gb of ram is not overkill.

How about this:

AMD FX-9370

8 Gb DDR3 1866mhz

AMD Radeon R9 290

Windows 7

More than enough to run today mmo.

Rather invest on:

27" LED monitor

Good gaming mouse and keyboard.

Good headset. maybe even wireless.

No need to go second rate with amd. If you want to save money, go i5.

Oh also on the power supply, make sure that it's a good quality branded PSU, not some generic crap. That's one component it's definitely worth going for good quality and paying a bit more. I prefer Corsair or Seasonic power supplies, very reliable. A cheap generic one won't last anywhere near as long, and higher risk of failure. Happened to my OH once with a generic PSU (this was the PC he had when we met, it was NOT one that I built!), the PSU blew out and melted most of the other components, the only salvageable component was 1 hard drive - so buying cheap can cost you more in the long run. That was a fairly catastrophic failure which is a worst case scenario and unlikely to ever happen to most people, but it taught me the value of a good quality PSU.

I usually go with Thermaltake, but yeah, don't go generic. You'll kick yourself when it fries your rig.

Not tried Thermaltake, but I will do some research and maybe add them to my list

There is nothing wrong with AMD and truth be told he would never know the difference, other than it ran cooler, O.C a bit cleaner.