It's not your RAM, I'm five-boxing with 3GB no problems at all. - EDIT: Well that's not true I have 4GB but it only uses 3.2GB (32-bit Vista).
As for your graphics card I can't say since I've never used that one, but I do know that I couldn't run four instances of WoW comfortably on my computer using only two cores, when figuring this out I switched to four cores and problem solved.
Best bet I guess is if someone here is using a dual core system for five-boxing and can vouch for it, though in my experience it's just a lagfest, a quadcore is needed imo for comfortable playing, but then again that is very subjective.