What you should be trying to avoid would be two things

1) reaching a memory used value that is greater than the amount of ram you actually have, which with 3 gigs shouldnt be a problem even with 5 clients
2) getting to a point where your main window lags so much that it isnt playable. I personally find its best to have ALL windows not lag (major towns being the exception) but that is just preference to me and aids in movement and such. You should try using wow maximizer because you can limit the FPS each client uses.

I wouldnt expect to get 5 clients out of that machine though, even with them all minimized.