I use 17" widescreeeen on the laptop plus the 21 for the cintiq, I do all work on cintiq tho, max, ps , bodypaint. and a third ,crt, for ref\movie\dl and color thingy, like ror said. hehe.
21"trinitron and a 17"lcd to the left of it. I work on the crt and have all the rest on the lcd (application pannels, winamp, my tv (dscaler)). And don't forget a decent multimonitor app; I find UltraMon is worth the money...one hotkey and your mouse cursor is on the other monitor. Things like that are really convenient.