i have one of the crysis pcs wich can do all in full
my pc specs for first run lfs was a amd 4000+ @ 2,4 GHz, 1,25 GB ram and the good ol' radeon x800 xt.
lfs ran good on full details and 4x AA @70 to 90 fps...
so this configuration is like the optimum for full graphics experience (thanks scawen that you are not one of the developers who thinks that buying a new game should be part of buying a new pc
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)
my pc specs now:
intel core 2 duo e6750 @ 2,66 GHz, 2 GB ram, 2x geforce 9600 GT
lfs runs really fine with the unofficial highres textures @ 150 to 200 fps