I thought I should revive this thread for the sake of more info for people who might need it.
The time has come, I've replaced my system. The new one is based on E8400 Core2Duo 3GHz (P43), GTX260+ and 4G@800MHz RAM (3.3G actual, still on XP32/SP3). Even though it has a nice OC potential, the system runs at stock everything, cool and very quiet.
Using the same LFS settings as my previous post, I can now get 150-300 FPS, and usually more than 200.
When I max all the settings (16xQ FSAA via the driver, X16 AF, best LOD- had to check the manual for that, no MP speedup, high res shadows, paths, etc), I get around 80-90 at the back of a large grid and 150-200+ most of the time during the rest of the game.
When not testing, I use vsync and limit the FPS to 70 and I get a consistent 60 FPS without ever dropping a frame. Nice
LFS' CPU usage is 20%-35%, and about 25% most of the time. It does seem to use both cores though (Using Z15 patch and latest NVidia drivers), although not completely symmetrically. One core is about 20%-30% and the other about 20%-45%. I'd say it's good enough.
As suggested by my previous and current benchmarks, LFS would cope way more than enough with much weaker CPU than E8400 (X3 weaker probably), and I think that a decent GPU is contributing more. Still, at its current state, I guess GF8800 (or equivalent) would be enough for very high visual settings at very good frame rates.
Only problem with my current system is that I now started playing Grid once in a while (great fun! very dumbed down physics yet still feels great), and it completely destroyed my LFS skills... My FZR just keeps spinning and overheating the tires!
Die Grid Die!