I'm running five on one-box.
Keyclone software boxing & Jamba for my multi-boxing addon; both are amazing.

I'm playing on a 30.5" wide-screen monitor.
Basically my regions are 640x480 (x4) along the top.
And then 1446x1085 in the left corner for my active window.

That leaves a fairly large window on the right for Firefox to check quests, or to play a dvd in to make farming tolerable etc.

Vista 64-Bit, Service Pack 1
Q6600 @2.4 Ghz
Sapphire Radeon HD 4870 X2
8 GB Patriot Extreme DDR2 @800 mhz
Raptor 10,000 rpm, 5 wow clients SymLinked from my main install.
G15 Keyboard for 54 extra macro's is nice.

The game seems to run fine, with graphic settings on (medium - eye candy / maximum - view distance) for my main window and minimal on all four slave windows.