The GT 630 recommends a 300w, but you might be able to get away with that. It'll run, just not entirely safely/stably.
I tossed a Zotac GT 610 into my brother's OEM desktop, which has a 250w psu, and it's never had problems.
But like the people who may post after me will say, it boils down to the amperage on the PSU. You could give us that info and we'd be happy to determine what's safe or not.
Hope my insight helps!