I'm running almost the same config as you:

5 boxing:

q6600 @ 3.6 ghz
8800 ultra
4 gig ocz ram
raptor main disk, samsung spinpoints offdisks.

I ran 5 instances on it first, 1 on my main 24", then 4 split into 4 on my 2nd 24".
I also had lag in shatt, usually 1 client with 1 fps, all the rest just fine (no idea why but whatever).
Instead of buying extra pc's I used my laptop to offload 2 clients on

laptop specs:
2.4 ghz dual core
7950 ultra mobile
2 gig ram

As for screen I still run my main on my first 24", then i have a vertical spliscreen of 2 alts on my right 24" screen, and another vertical split on my laptop (17").

With this setup I don't have lag anywhere. My main is usually at 60 fps, in shatt or so at about 30-40. Raiding environments its a lot lower (15-20ish) but I'm prety sure that's beacuse of addons and such, because with just 1 wow open (when i raid on my main) I have the same fps. (playing on max settings on main, bout middle settings on alts)

So basically what i'm saying is, 3 is not even necessary, 2 will work just fine, lagless

edit: about synergy I don't know, I personally use keyclone combined with Multiplicity, which works just fine