How can I operate 2 XServes and 2 iMacs from 1 keyboard and 1 mouse (both bluetooth)?
I use a combination of synergy for my laptop (macbook pro), and workstation (ubuntu linux). To get to my servers (some freebsd, some linux) I use just SSH, but if I need an X session I can export it over ssh. As Lyken mentions, ARD is the way to go. VNC is more crossplatform, but since you're running all Macs, its a piece of piss to get working. 

Synergy would be worth a go with your two iMacs. 

With that setup, you won't need any extra hardware, just a working network.