PSU details

[computermod14]

I know all the other parts of the computer like the back of my hand, but I dont know much about the psu, perhaps one of the most important parts of the pc. what should I really be looking for in a psu? the setup im going for is...

athlon x2 4400
asus A8N32-SLI deluxe
2gb cosair xms series
7800gtx
80gig seagate baracuda
120gig western d.

thanks for tips!