If you make two machines you can use a KVM switch to have 1 mouse, 1 keyboard, 1 monitor, and 2 pc's.
For Ridlyn, SLI you need 2 exect, and i mean exect video cards, like even a lets say there was an MSI 7900gt and an Asus 7900gt that wont even work they need to be the same everything.
Media Center i dont like for gaming so if your gonna use that computer id change the OS.
Personally id pool up my money make one sick machine. but you the machine you have is fine for both personally i think. i run auto cad 2006 on my PC listed below even before i oced it. but build the one super computer.