Do I need a 450w PSU, or will 400 do?

There is a local PC store that carries only 400w PSUs. Didn't get the model, but if it's a Corsair or Cooler Master, I'll probably pick one up.

However, will a 400 watt power my system?

I have an AMD Phenom X4 965 black edition, 4GB Mushkin Blackline, Sapphire 7770 GHz Edition, Optical Drive, and a 500GB HDD. Motherboard is an Asrock.

http://pcpartpicker.com/p/pIy7 - Drive and HDD are same brand as on PPP, but different models than what I actually have.

450 to be safe. gpu's eat power these days

400 is more then enough.

I'd say 450w or up, then make sure it has a single 12v rail and that it's 80+ efficient

I would go with 500 or a 550, I know that is overkill; but it will make it easier for upgrades later down the road

I was going to order a CX500. Sapphire's box says 450 at least, but I wanted to see what the community thought. My computer sits lonely on the floor, cold, without power. Why? Because fuckin' Antec. Dead PSU. 

Will it work, yes, i have a 400watt running c2d7200,3 120mm fans,3hdd,ssd,odd,and a 550ti. overclocked @20% and no problem(knocks on wood). However i do consider it my weak link and a 550watt or better is my next purchase, so if you are buying a new one just buy it once 500watt or better and 80+ is only a few more bucks dont settle out of desperation