I'd say if cost is an issue, going to a 1156 based core i5 750 is probably a good deal over the difference going to a 1366 based i7 (920), which is only about 20% faster in typical use. Where the $100 from the 5850 to a 5870 is a better boost for the cash. Building my son an i5 with a 5870 now. Had to drop the SSD from the plan, since the video card doesn't fit in his old case
Here's the specs:
Case: COOLER MASTER ATCS 840
PSU: Antec EarthWatts EA750
Motherboard: GIGABYTE GA-P55A-UD3
CPU: Intel Core i5 750
GPU: SAPPHIRE 100281SR Radeon HD 5870
RAM: G.SKILL 4GB (2 x 2GB) DDR3 1600
HDD: Western Digital Caviar Black WD6401AALS 640GB
DVDR: HP 1260i (SATA)
I had wanted to put a Corsair SSD in there with the HDD, but only have about $900 left budgeted (already bought the vidcard) and needed a bigger case than the one he had, 1/2" too small, and I don't want to cut the drive cage out.
Worth mentioning, if you plan on going crossfire, you will want to spring for the i7 on a 1366 based socket, that will raise your price about $300 (about 200 more for a good motherboard and about 100 more for the cpu). Any P55 board is unsuited to crossfire. Personally, I think that the 5870 is pretty well suited as-is, and if you *REALLY* want a boost, you can do a 5970 for $200 more, that will run about the level of 2x 5850's, and lower cost, with less issues. I've run multi-gpu setups in the past, and will generally have varying issues. With multi-gpu single slot boards, better to spend the money there. Also, I honestly don't feel it's worth nearly twice the price for 1366, if you run single slot video.
Doing the rest of the order from newegg in the morning when my check hits the bank. The vidcard is about $409, and the rest will be about $900 with shipping. so < $1350 total, and another $120 or so for an oem Win7 Home Prem x64.