I wouldn't say is overkill, perhaps more then enough. I would suggest go with your current setup choice. Really recommend the windows 64 bit choice (and yes vista is fine). The 8GB as some suggest is overkill for 3 accounts.

See, if you ever decide to go with five accounts, you'll have a system that will support it. You can always add more RAM at a later time, if needed. A CPU or a graphics card you would have to replace ;-)