3 boxing would probably be a much better alternative until you could afford a better machine. I hate to be the guy that comes in here to say "You can't do that" but, I'm not going to lie and say that 5-boxing on a dual-core machine and a 5 year old video card is going to be anything exciting.

Like I said, I don't exactly know what type of frame rates you would get out of a machine like that in Cataclysm but, really your best bet is to just try it out for yourself. I'll assume you have at least one active WoW account so, go create 4 trial accounts and see what the result is when you stroll through a major city.

Some people can handle playing WoW at 20FPS and don't mind entering major cities with only 2 or 3 characters at a time (out of a full group of 5), however, I am not one of those people and I would just not multibox until I could afford an upgrade.

Your CPU is the major limiting factor so, you would need a new CPU which would require a new motherboard. At that point you'd be limited by the GPU then.