I used to run 4 clients on one box with a very similar setup with 4GB RAM.

Performance was relatively good with moderate gfx settings, apart from high load environments such as shattrath. Performance was especially bad (i.e. unplayable) in high load situations within Alterac Valley.

I think your cpu and memory need attention in order to cover the above + wotlk (Wintergrasp pvp is a real beast on the single boxer, I'm struggling to get it smooth on a core i7 running 4 clients).

An upgrade to 4GB ram (with a 64bit operating system) and a quad core cpu would put you in good stead for running 4 clients in wotlk.

HTH