I'm not sure how much you want to invest in a new machine.

I upgraded from a similar machine.
Well kept the same hard drive and video card, case and power supply.
But got a new motherboard, ram and cpu.

My i7 920, with 12gb of ram runs 5 toons very smoothly pretty much everywhere.

You could try something like 1-2 toons on one machine and 3-4 toons on another.
But if you upgrade to an i7 or the equivalent, you might be able to run everything smooth on the one box.
Especially if you went with an SSD, which can be a huge improvement.

I'm going to go with a generation 2 SSD and windows 7, as my next upgrades.