I don't have any direct experience with the Pentium D, but I'm skeptical that you'd have a good experience running 5 copies of WoW on that system. I have a 4GB Core Duo, and I found that I need to upgrade to a quad proc to get acceptable frame rates with > 3 WoWs. The 2.4 GHz Quad Core Q6600 is going for like $250 US right now. You might consider dual boxing for a while on your current machine until you can save enough to buy something more up to date.