CPU shouldn't be the problem for a single card but...
I have a c2q 9450 @ 3.2 (stock is 2.66) and DDR3 1333 RAM and 2 x GTX 560 SoC (super overclock).
With one card, my system was not bottlenecked, but in SLI I only saw roughly a 25% increase in performance while my CPU was at stock speeds. Once I overclocked my CPU to 3.2 I saw another massive 15% or so increase to performance (I tested using Crysis and 3DMark Vantage). I still feel my CPU is keeping me back, so I'm looking forward to Sandy Bridge upgrade very soon.
All that being said, Big Mac, if you play alot of multiplayer in BF3, then a new CPU will help heaps. Multiplayer in any game uses much more CPU power then single player