I've got good news. All you need is an old PC (I use a P-II 400mhz w/512mb RAM) to run a web server. I have a sever P-III dual 500mhz w/1 gb RAM but the old P-II does a good. I use Windows NT 4.0 SP6a SERVER on the P-PIII and W2K Pro SP4 on the P-II.
I am not trying to talk you out of it but a home web server is a low power computer. You choke point is not the computer but the upload speed of your DSL/Cable/ISDN/T-1 connection. In my case I have cable, and am limited to 768kbs upload. So either of my computer are more than able to handle the requests. At DSL/Cable Web Serving
you may find some tips that can help.
Whatever you do do not put anything on this computer you cannot afford to loose. The chances of you being attacked goes up since you are inviting people to visit you computer via HTTP or FTP. Only turn on the services you will be using. If you are not going to use FTP make sure it is off. Stuff like that.
I have read where people are still using 486/33mhz with 32mb RAM to server a site. If you only have 64 or 128kbs ISDN that is all you need.
What type of connection and how fast are you connected? Up and down.