Why are you building one massive machine when 4 low end ones with enough memory, a decent CPU and even just an 8600 video card plus a central fast machine would most likely beat anything you can throw at it on the market today? All SCSI is going to do is slightly reduce load times when you enter an instance. That's about it. Sure, there will be some marginal gains but nothing that is going to make you go HOLY CRAP.