For smooth multiboxing many suggest 2 clients per core max, 1GB RAM per client min, and an SSD.

Obviously you can multibox with less than that, but for smooth multiboxing it seems to work. I only ever lag at the end of a WG battle and I feel it's probably due to server lag, not my computer:
i7 920, 12GB RAM, SSD, GTX275 1.8GB VRAM