I would suggesting upgrading to an Am3+ motherboard, get DDR3 memory, and windows 7. If you have room in budget, you could sell your current video card and up it to a 6850 or 6870.
JareeB, I'd have to say that your HDD is probably making the system slow. It's fairly old, only 320GB and a Caviar Blue drive. I'd suggest an SSD + Samsung F3 1TB or just the Samsung drive if an SSD is too expensive.
And windows 7
If you want to max out BF3:
990FX Mobo, use your athlon until Bulldozer is out. Or buy a new CPU now. Up to you.
8GB of 1600/CAS9 (CAS8 prefered) DDR3
GTX460/560Ti/570 or Radeon 6850/6870/6950
Your PSU should be fine for that as well.