500W is plenty for a P4 set up
-Your motherboard supports (correct me if i'm wrong)
-6 x IDE HDD/ODD
-4 x SATA HDD/ODD
-1 x IDE FDD
You have 3 IDE controllers plus a floppy controller. Plus you also have 4 SATA controllers.
I attached a picture of your mobo and highlighted the different connectors/controllers
Edit:
The difference between the two HDs is that one is OEM and one is Retail.
OEM-The hardware is all that you get...no retail box, no software, no extras
Retail-Comes with everything...manual, software, drivers, cables, retail box, etc.
Ask yourself if the extras are worth the difference in price. But an OEM with a 3 year warantee I would take anyday over a Retail with a 1 year for $30 more.