Not a very good power supply overall, and also below the minimum capacity that's specified for the GTX 650ti. You SHOULD have a LEAST a 430w unit that is of good quality. That unit isn't a firestarter, it's not THAT bad, but it's build quality isn't great and it's totally not intended for use with a full time gaming card.
http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story6&reid=341
Plus, and maybe more important, it's a group regulated design which does not tend to play well with the Intel C6 power state that's used by your CPU. That alone could be affecting operation, although I think it likely that would only affect sleep modes and not be a factor in it not shutting down. Still, it's pretty high on the list of possible issues at this point.
I would definitely try resetting the bios, first, by pulling the cmos battery while the unit is off and unplugged, then resetting the bios to default settings.