I just bought BF3 for PC and it runs flawless. Except I have a mildly OC'd HD 6870. I am not sure about how your GPU's perform but I assume well if you plan to keep them for now. The only thing I would recommend is put some emphasis on cooling unless you already have a well ventilated case. Your blueprint looks to be much better than mine with higher end tech. I believe your rig will run any game flawlessly since mine does except those pesky buggy games like GTA4 (that's the devs fault!) If you care to read my specs here they are. I play Rage, Dawn of War 2, Crysis 2, Guild wars 2, Shogun Total War 2, Empire Total War 2, BF3, Mass Effect series, Dragon Age Series and more all with no worries. You're good to go!
Here a video that shows your cards at work with BF3 and they look like keepers! Flawless.
AMD FX-8120 8-Core Black Edition Processor Socket AM3+
An old Western Digital Blue 640 GB HD from like 2008
XFX ATI Radeon HD6870 1 GB DDR5
Diablotek DA Series 600-Watt ATX Power Supply (cheapo)
ASUS M5A78L-M LX PLUS AM3+ AMD 760G Micro ATX AMD Motherboard (not high end at all but I didn't spend extra money on stuff I am not going to use like USB 3.0 over 8 GB ram, more than 2 SATA slots or SLI/CXFIRE)
Kingston Technology HyperX 8 GB (2x4 GB Modules) 1600 MHz DDR3 Dual Channel Kit (PC3 12800) 240-Pin SDRAM (New BIOS update got it working at 1866mhz OC)
NZXT Technologies TEMPEST 210 Massive Mesh Style Midtower Case (with all NZXT 5x120mm and 1x140mm fans. No cooling problems at all.)