I don't really think your C2D E8400 will die cause of 4 or more WoW clients.

Am running a E8400 with a GTX 260 and 4 GB RAM (Win XP) without any problems for a couple of months now. I don't think a 5th client would kill my CPU or cause extreme slowdowns in gameplay... (am just 4-boxing cause my wife is playing her account to complete a group for heroics)