Because the Alienware X51 is a small form factor prebuilt case, it won't fit regular PSUs.
@the1566
It's pretty simple to do like the Overclocker's tutorial explains, but I don't know how to operate the 2 PSUs together (in order to run the GTX 670 with the start of the PSU in the original case), for the GTX 670, you just plug the PCIE connectors, close the circuit on the PSU, power it on from the built in switch or make an external switch like he did.