I'd get either the CPU or a second GFX card, depending on if BF3 will have good Crossfire support or not.
SLI and Crossfire tend to be somewhat buggy when a game is first released, until everything can be tweaked to improve it. I dunno if a 6-core CPU will help you -that- much... I guess you could dump most of the windows processes to one core and free up the rest for BF3, which might help.
Obviously it'll also help if you can overclock the crap out of it.
Looking at the prices, I'd say maybe grab the CPU. It's a bit cheaper than a second card and you might be able to overclock it nicely... which will help.
60GB (120GB WD Scorpio Drive) PS3 (CECHA01, NTSC), 3.7OFW
60GB (640GB Samsung Drive) PS3 (CECHC04, PAL), Kmeaw 3.55, Repaired (Dead Laser)
PSP-1003 FAT, CFW5.00 M33-6 & Prometheus v2
X-Box 360 Xenon (JTAG'ed, 500GB HDD)