I know this is a vague question. But here goes nothing,
Assuming i have a P4 Hyperthreading 3.0 Ghz, 1Gig of ram. Running Mysq/Php/apache!, how many wordpress installation do you think i can fit in there assuming they are all doing normal blogging load? (i donno, some thousand views a day) Oh yeahh, no heavy plugins, and this machine WOULD NOT be dealing with any media, vid etc?