I think the disc that Vista is on have both 32 bit and 64 bit. I think it detects your hardware and if it's 64 bit compatible, then it will load the 64 bit extensions onto it. If someone knows about this, correct me if I'm wrong. Microsoft is aware that not all PCs are 64 bit ready so I'm sure they're prepared for that.
Although...I'm tempted to give it another shot. If it locks up, I know it's the board or CPU. But...I won't try. I don't want to trash a such good working OS.