It's more than possible, it's just the FPS that you get that's the problem.
I know a couple of people with OC'ed Quad's (ie 3.6+ range) and SLi'd GTX's who are maxxing it, however most people with PC's that fast tend to have giant monitors that means their system is under much more strain that the average person.
@ Zoomy - No idea how GT4 works resolution wise, but LFS will look as good as the TV lets it as it will run at the right resolution. "HD" is a term bandied around by a lot of people that don't understand what it is, and doesn't really apply to LFS at all.
To ask that LFS get no heavier on a PC is stupid imo, it's a DX8 game that i can play using a PC with a CPU under 2 GHz and integrated GFX - you CAN'T expect a game to run on a PC from the ark, and you can't expect the devs not to develop something to stop them alienating a few customers. In sim-racing, a niche, you can't afford to require DX10 and multi-cores, but you shouldn't need to really support anything lower than a Skt A 2200 and a 9600.