For $750 that build has good "bang for the buck". What size/resolution monitor will you be using?

The case/PSU combo is a good one but the PSU is a bit under powered for two 4850 cards in Crossfire (550W recommended PSU).
For not much more than that case/PSU combo you could get the CM Centurion RC-590 $50 and OCZ StealthXStream OCZ700SXS 700W $60 (after rebate) or CORSAIR CMPSU-650TX 650W $80 (after rebate)

Spending an extra $125 to upgrade to the Q9550 2.83Ghz quad? For gaming you'd be better off getting a 4870 GPU and get a good CPU cooler ZEROtherm ZEN FZ120 120mm CPU Cooler $44 and $30 rebate and save about $90.
Overclock the Q6600 to ~3.2Ghz and you won't give up any performance over the stock Q9550 or i7 920.
And the 2 PSUs I suggested should even handle 2 4870s in Crossfire.