I think you could do 4 accounts. 2 Accounts on 1 core each. Or main on one core and 3 other on 1 core. But you will lag 24/7 and your pc would die in Dalaran.

If you get a quad core (a lot of multiboxers including me have Intel Quadcore Q6600, and it works great) i think you could do 5. Or 4 with much more performance. Also with 5 boxing my RAM was constantly 99% in use, now i got 8 GB ram and i had around 60% in use.

But Alterac Valley or anything high quality like dalaran wasn't playing great. Had around 15-20 fps in the midfield of AV, so i decided to 4 box. But with leveling my toons i had like 40 fps on main So that was great.

Kind regards,

Steven