He has a custom build not an OEM system there. greg. Clean Install Windows 7
But for replacing the present OS installed you would want to do exactly as outlined before as far as nuking the OS drive clean in preparation for a clean install of 7 either by a usb install key or by dvd. You would to install 7 on the intended OS drive not simply to another and then dump the other OS.
Besides that you may want to consider the 32bit Ultimate over the 64bit due to the system specs you have there. On one old beater case with only a pair of 512s for memory and a 128mb AGP card plus onboard sound I tossed a temp install on the old 250gb ide of the 32bit Ultimate without a problem. I think the 64bit install might have also run into a few problems on that old case.