It also depends on if you want a managed, semi-managed, or totally unmanaged box.

I'm going for a managed server (because right now I don't feel like my server admin tasks are solid enough to provide really good client support, so I'm "keeping the training wheels on" for a little while longer), so I'm paying more per month - but I know the network team, I know the quality of work they do, I know the quality of support they provide, and I'm more than happy with it. In a few years, I may be able to suggest moving to a different environment where the servers aren't managed and our monthly per-server costs will be lower - but until my admin-fu is much stronger, I'm sticking with a managed option.

You'll also pay less if you get an EIDE drive versus a SCSI drive. I've been trying to find someone who's used both in a shared-hosting environment and can tell me the actual differences they noted, in terms of performance; but thus far no one's talking.