Well i have a:
-Pentium D 3.0 GHz
-Gigabyte 945 Mother Board
-WD 250 SATA 7200
-2X512 CORSAIR XMS2
I run (Call of duty) and (NFS most wanted) on full settings and every thing is fine.
sure i want a bigger PSU, but this one is ok.
I think a 400watt should be fine.