Used to run it on a P4 2Ghz with 512MB RAM (XP Home 32 bit ofc) and a GeForce4 MX460.
I got somewhere in the vicinity of 40-50 fps I think. With an empty track that probably would be. As people before me have mentioned, LFS is a lot more hungry on the CPU side of things than on the GPU side. Getting one of the newer high-clocked (or easily overclocked) Dual Core's should net you a nice fps increase if you're desperate for more. Or phrased differently, LFS probably would run best on a high-clocked Dual Core.
To be fair, over the years, LFS graphics have always looked neat, and there's been some upgrades, but the general shape of things hasn't had any major overhauls. (Unless I'm wrong of course, which I can be).
-Z