It has 20A on 12V rail. It's on the edge, though it should work. It's a crappy power supply, you should replace it eventually even if it powers the card up. It could blow up at any second. Get a quality Corsair, Antec or Seasonic unit.
Even though I prefer nvidia, but their cards literally suck in low budget range below GTX 460. HD 6770 is way more powerful than GTS 450 even at similar price.