I had 7 computers (plus TV, Amp, cable box, NO lights) on one circuit, and it'd trip out once a month. Usually happen when I was zoning and all the hard drives kicked in.

I went to add an 8th computer (high end, 750 watt etc) and had to move 3 of the low end computers to a different circuit. I used a thick extension cord and ran it from the other room. It would get warm, but never hot.

If you're serious about this, I would suggest removing any unnecessary hardware from the bot computers (mainly the CD-ROM drives). If you are spec'ing out new computers, keep power usage in mind. Not only can you run more computers on a single circuit... it will cost less in the long run as well!