For the small monetary difference i would go with 400 watts. The burner most likely acts as a cdrom as well and you can buy them that way as well. Ram depends on the motherboard you get. The harddrive will work as long as you format and reinstall. It cannot act as master if it has a different computers hardware prtofile on it, but otherwise its useable. Hope this helps.
Windows XP PRO:$0, Visual Studio 6:$0, Age of Empires 2:$0, Halo:$0.
Having $100s in free MS software while people are being sued over $.99 songs: Priceless