With a 3800/7900GTX combo, the limiting factor would be the CPU as the GPU is one of the very best available (and the CPU isn't)
Um, no. Getting a 4400+ or even an FX60 won't improve anything, maybe will add couple of framerates. Playing at high resolutions (1280x1024 at least) and playing at high settings, eliminates the possible CPU "bottleneck" since the GPU is doing most work, so the CPU doesn't.
I would say 3800/7900 is just fine. 3000/7900 is still good enough since the CPU has little less MHz.