all depends on what load you're expecting. If it's just for use at home where you're the only ever user and the apps are light, an old P200 with 128MB RAM will be ample. If it's to be a high volume internet server with thousands of consecutive users and heavy weight applications running on it you're looking at a multi CPU Xeon machine with several gigabyte RAM.
Most likely your requirements are somewhere in the middle
Do the next thing next. That’s a pretty good rule. Read the tiny ad, that’s a pretty good rule, too.