let's see.. an AMD 4X4 system, with four of AMD's top quad core processors once they come out..
I'd say 4GB RAM (1GB stick per quad core chip.. or 2 x 512's per chip if they support dual channel individually) - I'm not going to use more than that now... and more would slow it down probabbly
and also, 2 x X2800 XTX's in crossfire when they come out
oh, and a fibre optic internet connection
you know, I'd most likely end up running about 12 instances of folding@home on it at once, and leave 4 of the cores for other stuff... I'd still have extreme multitasking WHILE being an insane folding@home contributor