First go make yourself some trial accounts and see what your computer will handle. I would try goign to a major city (if your horde make some orcs and goto ORG) this will kind of let you know how things will go in the outlands. So this will tell you what your computer is capable of.

I tried 5 toons on a macbook laptop. It was super fun super laggy but really really fun. This let me know well if I want to do this then I need a major upgrade on hardware.