If you are planning to upgrade your rig in the near future then i would say go for the 650W. Then you wont have to buy a new PSU then. Otherwise, the Corsair CX400W is a nice choice and would handle your needs quite well. HD4650 is a very low power consuming card.
A 400w PSU is sufficient to run your system. I would however like to know your full system stats to confirm this.
And inline with what hell-storm said, If your planning on upgrading in the near future get a 650 ~ 850w PSU so you can SLi if needed, or atleast to support a GTX 480 or similar(AKA Power hungry cards) if need be in the future