Hi!!
Till last month i was using Intel Celeron 2400@3000, 9600pro 128 and 512RAM@333. When I was alone on track I was getting around 50 FPS. BUT when I adden like 4 aiit was going really slow(20 FPS). But you have to understand that LFS hase nothing to do with it. CPU simply couldn't run games(whole bunch of oters too). Now after mainboard failure I bought AMD Athlon 64 3500+(2200)@2500 Venice 939-pin and i make around 90 FPS alone and above 40 with 11 AI. I have only v-sync off. Other setting are full.
Conclusion: It's the CPU that slows you down, but if this is sudden then I believe its WIN.
I hope I helped someone that is asking himself abou buying CPU or VGA
EDIT: oh yes, I'm havving like 56 processes minimum and ony 204MB of free ram avalible