If you haven't purchased yet, I'd go with an i3-2100 and LGA 1155 build. It has better performance in games and can upgrade to a better CPU if you want (http://www.tomshardware.com/reviews/best-gaming-cpu-core-i3-2100-phenom-ii-x6-1075t,2859-10.html).
Of course, certain AM3 boards are confirmed to be Bulldozer compatible. So that is a viable route. If $10 makes a difference, you'll have better gaming performance with an Athlon II x3 (unlockable) and a better graphics card than a Phenom II x4 and worse graphics card.
This is pretty much the best choice for an AM3 board (ignoring Bulldozer compatibility, which I know very little about): ASRock 870 Extreme 3 (http://www.newegg.com/Product/Product.aspx?Item=N82E16813157198&cm_re=asrock_870_extreme3-_-13-157-198-_-Product)
Get the 1600MHz CL9 4GB (2x2GB) DDR3 because it's pretty much the same price and CAN give you 2fps. If you get 2fps here, 2fps there, you get 10 extra fps, which is $40 in graphics cards and worth $0 here and there.
PSU: Use Extreme Power Supply Calculator Lite (http://extreme.outervision.com/psucalculatorlite.jsp) to find your required wattage (15% capacitor aging). You should be around 360W and 470W in Crossfire. Then get a Corsair, Antec, Seasonic, Silverstone, or XFX power supply. This is a 600W Corsair for $50 (after rebate, free shipping)
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139019