The biggest resource hog in my experience is MySQL or any other database program. If you're just serving html and php then 1.x ghz is a fast enough processor. DBase queries can produce a lot of overhead in which case you'd do best to use two or more cpu's and at the very least, a gig of RAM. But that's just my experience, your mileage may vary.

-Seg

edit: optimally it is best to run your DBase on a seperate server.