SeaSonic 450W will be good enough for your rig.
If you can find the 512MB variant then go for it.
Cuz for your monitors resolution 1GB is not required .
1GB Ram comes in handy only if your are running 19X10 or 16X10 resolution.
Reading this might help you:http://www.tomshardware.com/forum/271940-33-5750-512mb-deal