I would still go for a higher wattage, I run a Corsair TX650 on my system, & as absic says, post/boot does use a lot of power, I have even noticed my temperatures to go very high at this stage.
To be on the safe side it is better to err on the upper than lower wattage, plus later on you may plan to update your equipment, or add more hardware.
I also agree to recheck all connections, have you tried to run in safe mode or can't you get that far, if you can it could be a simple driver problem or conflicts, just a thought.
Ben.
Happy 2013 to all.