I do pretty much exactly this.

I have my MBP as my main, with my main char and a herbing alt on it
I have a G5 iMac with 2 chars on
I have an old AMD64 pc running with one char on it.

I use clonekeys on the macs, and synergy to broadcast to all of them.

Works very well for me, and didn't require any hw outlay, which is prolly what allowed me to make it happen