balrog I'm 5-boxing on a laptop too. It's definitely a passable solution for getting started, especially to find out if multiboxing is what you really want to do before throwing down the cash for beefier hardware. I'm still just getting started myself and can take the less than optimal performance for now.

I'm running on an older version macbook pro:
2 GHz Core Duo (that's right, pre-Core2)
2 GB ram
Radeon x1600 w/ 128 MB

I do have graphics setting all the way down on all 5 game instances. The 4 client windows run on a 24" screen at 960x600 each, the main window on the laptop screen at 1440x900. I have fps capped at 8 on the 4 clients and 20 I think for the main.

Using CloneKeys and running all 5 like that, keystroke broadcast is lagged to the clients and one will often not get the key. Walking through Ironforge is like flipping through a picturebook, slowly, unless I tilt the camera to directly overhead and zoom all the way in on the 4 clients, then it's not so bad.

I found, however, that if I only 4-box it, everything is nice and smooth (though I haven't taken just the 4 to IF yet). Keystrokes always make it to each client, without any noticeable lag. I can increase the max fps on the 4 windows and still have the performance, but don't really feel the need to.

One essential item for multiboxing on the macbook pro, so nice that I use it all the time now, is a program to manually control fan speed. Mac settings value quiet over cool. On the older ones especially, these things can get seriously toasty before they'll kick up the fan rpms. If you can find a way to bump the minimum fan speed on your Dell, your CPU will be happier.