I've been really happy with EV1 Servers. I have a P4 2.4 w/ 1GB RAM and 2x60GB HDDs. It's almost a year old though so we'll probably be upgrading the processor and ram pretty soon to P4 3.06 w/ 2GB ram.

It's really fast and the support (for unmanaged servers) is really top-notch. I've been with them almost 2 years now and don't have any complaints.