So your new 650W PSU works in one computer but not in the other? Are you hooking up all the cables that are necessary? Graphics cards sometimes need cables, the motherboard often has multiple cables, etc. etc. Sorry if I'm being redundant but if the PSU is indeed functioning properly in one machine, then you must not be hooking it up properly to the other machine. What graphics card/cards are you using in the machine that won't work?