My 1st system has a single core AMD 64 4000+ & my dead system has an AMD 64 X2 4400+.Now the question if I were to swap out the single core for the dual core will I have to reinstall Windows

& all of my games again).I have 3 Hard drives 1st is OS, 2nd is Music/Media, 3rd is Games.Any help would be appreciated.
They are the same socket so you can swap them, and no, you shouldn't have to do anything except put it in and start it up, it's possible that you may have to re-register with Microsoft to verify you haven't pirated the copy of Windows because you have switched out hardware but that would be about it.
Keep in mind this is how it SHOULD work, I can't guarantee anything, but your hard drives should definitely not be effected unless you effect them in a settings change.