I run 4, sometimes 5 box on one computer, usually at 75% load of my PC, in the evening busy hours in shatt, I experience client loss occasionally and instead of mounting through I tend to walk through which seems to work.

Shattrath and the AH in Orgrimmar so far are the only two places I have experienced this amount of lag. I rarely ever lag anywhere else.

Got a core2quad 2.4Ghz, 4Gb ddr3, EN8800 ultra 768Mb.

I run my toons in a 4 box at near max settings, each WoW assigned to their own core.
I run my 5 box with main at max settings and the other four at low settings, running the four alts at a core each and no core assigned to my main.

Shattrath at non busy hours is like anywhere else though, no lag for me.