Well, the good news is that you probably don't need to upgrade your monitor. :-D

The bad news is that you are not gonna be able to effectively run 3 or 4 characters on that machine.
Ideally you want to upgrade your Motherboard, CPU, RAM, Graphics Card.

For running 4 characters on that machinem you'll probably want a Quad core processor (Intel Core 2 Q6600) Around 4GB PC6400 RAM, 512MB 8800GT.

You''ll probably need to upgrade the power supply as well.

Its not cheap but it'll perform, which is whats important.