Just a note: The first two SATA ports in the XPS 8300 (labelled SATA0 and SATA1) are the quicker SATA 3.0 (6 Gb/s) ports and the remaining two ports (labelled SATA2 and SATA3) are the regular speed SATA 2.0 (3 Gb/s) ports. You can see it in the Dell service manual here: http://support.dell.com/support/edocs/systems/xps8300/en/sm/techov.htm#wp1268982
So I would definitely connect an OS to one of the quicker ports. This is for the XPS 8300 though. In the service manual for the XPS 8100, it doesn't differentiate any of the four SATA ports (it just lists all four as "SATA connector"). I'd probably still put an OS in port 1 though just to be safe.
