Definitely your CPU. I had an Athlon dual core, when I bought my GTS 450 a while back, I was bottlenecking badly in Battlefield bad company 2. My board was an AM2 board, probably like yours, but with a bios update, allowed me to upgrade my processor. I dropped an Athlon II quad, which are dirt cheap now, I think they are floating around ebay for under 75 bucks. And the PC really came to life. My PC is 5 years old, next upgrade for me is a better graphics card and power supply. Though I do want a 27 inch monitor also.
Check your board's specs, if you can drop a quad into it, do it. You would be suprised how your old PC roars to life. Soon as I put my quad in my PC, bottlenecks were gone.