those mac you your seeing around are just workstation consoles, and there not cheap, for the same price (but probable much cheaper) you can buy a server/workstation board, that can house to clovertown CPU's, this is the same way MACs do it, you'll also haev all the availbe RAM modules, which you can construct up to 16Gb. Many of the boards have PCI-ex X16 so you put in a decent/bussisness GPU.
The advantge of this is you could run Windows XP x64 or Vista x64 (becuase of ram usgae must be x64)
If you want one premade i think HP have one, im sure they have a duel woodcrest (2*2) so they should have ugraded to dual clover town (2*4).