I recommend 7850's. Only cost $100 each (I craigslist/ebay the games).

Im going to leave my math here, and head in shame. I just fucked myself out of free money.

2 7850's = $200 = 640 (+/- 20) M/hash
1 7950 = $250 = 540 (+/- 20) M/hash

That is $250 you save per rig. Which ends up covering the cost of another rig to make up M/hash

My 2, TWO, rigs = $1500 with 8 7850's = 2300 M/hash

When I get my riser cables I can add 1 more gpu per board. Which = $1700 at 3,000 M/hash

4 x 7950 = 2200 M/hash = $1350 on same hardware. Or 5 x 7950 = 2750 M/hash at $1600

Please note im calculating the games combo being sold for $50, into the price of both GPU'S.

Its 5 O'clock somewhere.....Errr 5 AM here. Im going to be pissed in the morning when I double check these figures to find out I cheated myself.

ALSO THE 7950'S ONLY REQUIRE 30 MORE WATTS AT DOUBLE THE M/HASH FFS.