Just on a side note, from my experience with running 7 clients on a Windows XP machine with 2Gb RAM and a dual core T9400 (I think), the killer is not the actual time spent processing the game itself, it's the loading and caching on the disk.

Make sure you let your first copy of WoW completely load and wait for the hard disk activity to come to a stop. Then start loading the other clients. By the time each one has gone to the loading screen I am logging in on the next one. Then, once the are all loaded, I wait a good minute for the hard disk to die down again.

Travelling to a city is the same. I used totally avoid them. Now, I put one into the city and wait for the hard disk to completely come to a stop. Then I enter the chars one at a time, with a little pause in between. I went from avoidance to walking 5 chars through a city no probem.

Logging out is a similar process. Make sure each one gets a clean exit and has completely closed before closing the next one. If I go too fast, they tend to crash instead of exit properly.

With my fairly crappy specs, I can run 5 clients no problem at all and 7 when I need to swap stuff around or trade to my banks. It just takes a bit of extra patience to let the operating system cache WoW's data fully before WoW starts thrashing the hard disk.