From what I understand, you purchased a SATA hard drive & an external enclosure & want to know if the enclosure is installable in the PC case's 5.25" drive bay?
"If so, what happens if I then plug in a cloned HD (also with the same XP loaded)? Does the computer challenge me to say which disk is the boot disk or worse, throw a wobbly?" - shouldn't make a difference, as the original OS drive will be already set as the 1st boot device in the BIOS. G