Dual CPU perform faster, but probably not more than you think, benefit for dual CPU is stability.
My config now(bought in 2 yrs ago):Dual CPU board(made in Tai Wan, model no. forgotton), 2 x P3 450M Hz CPU, 3 x 256M RAM, oxygen VX-1, Ultra wide SCSI HD, Win 2000(WinNT previously)...etc.
By the way, I don't know much about hardware, anybody can tell me about SCSI nowadays?
Try buying more ram b'cas its cheaper than b4, you also needed to spend more $ on display card for faster real-time modelling and rendering.
