A few weeks ago I upgraded from an Athlon X2 4400+ to a 6400+, and the difference at the start of a race, with 19 AI, was huge. 20-30 fps before, never below 50 now, avg 50-75 ,(at start, above 200 after that) same video card, all settings maxed, it's certainly the cpu.
Also, from experience, plus what I see on paper, a newer dual/quad core cpu, core for core, is far superior to older, single core systems. Many of us would notice this behavior, most software isn't multithreaded, and this software wouldn't run any better on a X2 system.
To OP: Myself, I would upgrade the processor, or wait and build/buy a newer system, good luck.