http://www.newegg.com/Product/Product.aspx?Item=N82E16813130679 $70
Searched for all AM3+, AM3, AM3/AM2 boards, lowest price with 2 16x slots, also has 2 1x slots that you can later add gpu's to with powered riser cables.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139028 $65
Searched for 550w supplies on up with at least an 80+ certification, first decent PSU on the list when sorted cheap to expensive. You'll also have to look at whether your gpu's have 6 pin or 8 pin connectors and how many so you know how many adapters you need to buy if the PSU you get doesn't have enough.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817182133 $90
This would be a decent compromise - 750 watts 80+ Bronze single rail modular. Easily run your current set up and you might even be able to squeeze a 3 card out of it later.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125414 $310
Actually just realized you were looking at the 7850's and not the 7950's!! Their power draw is much more modest. My advice on choosing a different supply still stands.
I would also advise getting the 7950 over the 7850 even though they are close, the 7950's are about $.10 per KHash/s cheaper and about .02 watts per KHash/s more efficient.
Your build right now totals about $445 not counting ram or the fact that you really should get a better PSU at the very least. My suggestions thus far with the better PSU would put you at $510 with 1 7950 and no Ram as you also left out and could get you about 650 KHash/s or 1.2843 KHash/s per $. Same setup but with your 2 7850's would cost $540 and get you about 820 KHash/s or 1.4818 KHash/s per dollar. This changes once you add more cards for example if you add a second 7950 which brings the build up to $820 and KHash/s to about 1300, your KHash/s per $ goes to 1.5976 which is now a 2 gpu setup compared to a 2 gpu set up as opposed to trying to compare build cost. Anyway, just my 2 cents... or more