My slave box runs 4 toons on a 23" monitor (1920X1200). It uses a 9800GT (or it might be a 8800GT, don't recall which one is in which). I have no issues in Dalaran or anywhere else. My slave toon graphics settings are set fairly low (no reason to do otherwise is there?).

Now, I don't PVP and haven't 25 manned anything, but a lot of the current setups I see posted seem way overkill for running even 5 instances of WoW. Technology has very much surpassed the requirements of a game like WoW.

Personally, I'd go for a 2 or 3 computer set up before I spent a lot of $$ on expensive components. This option is obviously better if you have older computers sitting around.

Anyway, I doubt you'll have many issues with a GTX280 =).