I'm using 4870 X2, Keyclone with 4 smaller regions along the top of the screen, a large region in the bottom left and a smaller blank space (Firefox/Vent etc) in a column on the right.
I've set my max fps to 8, for the smaller windows since you cannot read their text or mini-map anyway.
Pretty much the main window is huge (30.5" monitor) and runs the slaves fine.

I'm running:
Vista 64-Bit, Home Premium
Q6600
8gb, 800mhz ram
HD 4870 X2 (I updated the drivers from their website, in early Sept when I had the freeze issue, didn't solve it).
SymLink 5x WoW on 10,000 rpm Diamond Raptor.

I noticed I would very rarely freeze when landing on a gryphon. It would always be just one of my four smaller windows.
It could be any zone in the game, did not matter if there were a lot of players or none, old world or new.
This was pre-WotLK.

Turning the settings down for the slaves, via the SymLink, and the problem has gone away.
The main screen has pretty much every setting maxed and runs smooth anywhere.
Have not got to Dalaran yet, but Shatt was a smooth 60 fps with everything on, after the initial 2 seconds of loading latency.
I have a killer nic, not sure if that helps with the load times or if the delay is more hard drive related.
Howling Forde and Borean Tundra both run smoothly for my current set up.