Computers will usually last a long time, but you probably don't want to have it for that long.
I believe you should always have at least 1/2 the processor speed of whatever the top consumer pc's are running. That way you dont' throw a lot of money away, but don't have a pc that's too outdated.
Like right now, 3.2ghz is about the top, so a 1.6 or so is what I'd consider to be a minimum speed.