You need a router, first of all. The most recommended model among members of this site is the Linksys WRT54g. This does have wireless functionality, which if you're buying a router anyway, I would have thought is worth having for future use even if you don't intend to go wireless immediately.
The router requires ethernet ports on the PCs. If the PCs don't have them, you can either open up the cases and insert some (at a cost of about £10 each) or buy USB-ethernet converters at about £20 each. To save £20, I know which I'd rather do.
Hope that helps.