For your average webserver, you don't need very fancy hardware. Depending on the amount of traffic your going to get, bandwidth becomes the limiting factor. For example, I've run a webserver off a P166 running slackware linux, and the page loaded without any slow downs. Of course, if that site had gotten more traffic, performance would be lost.
Before you start building anything, you should figure out what exactly this server would be for and how much load it will get.