At $400 (+$100 for Win 7), I'd go AMD. If you had a little more money, you might consider an i5 build, but the only motherboard I'd go with is the Asus P7P55D-E Pro, which runs $180. That plus an i5 is going to be close to $400 already, leaving little budget for RAM.
That's good things right there. Thanks! I can probably work a i5 build for about 100 bucks more. Guess it depends on what the budget looks like.
That MSI mobo is good. If it cost a lot more than the Asus 890GX, it wouldn't be worth the added expense, but with that combo at nearly the same price, it's a good deal.
Right now, there are 2 AMD lines to consider. The 700 series and the 800 series. There isn't a lot in the 800 series that's worth paying a ton more for, but there are some future-resistant options that might be worth $20-30. The 800 series boards are more likely to have USB 3.0 & SATA 6.0 Gb/s, mostly. Those features are few and far between, if they're available at all on the 700 series.
Beyond that, there are the different models within each series.
FX is top-of-the-line, and does CrossFire at 16x/16x, roughly equivalent to Intel's socket 1366.
GX is the 2nd most expensive model. It has integrated graphics, but can CrossFire at 8x/8x, and is pretty equivalent to high-end socket 1156 boards.
Below that, there are multiple models for both the 700 and 800 series. By and large, the only other models worth considering for gaming builds would be 770, 870, and 790X. Most of the other models have integrated graphics and won't CrossFire.
Of course, CrossFire isn't necessary for most gaming builds, but as you get into the lower-end models, you also lose features. My typical recommendation for an AMD gaming build is an 890GX mobo, as it can still CrossFire if necessary, but you're not spending the extra $40-50 for an 890FX motherboard...but you still get the other features, like USB 3.0.