In a small business environment, is it still necessary to have a central server?

Speaking for my own company (a small charity with about 12 employees) we use our server (Windows Server 2003) for the following:

• Email via Microsoft Exchange
• Central storage
• Acting as a print server
• User authentication / Active Directory

There are significant costs associated with running a server like this:

• Electricity, first for the server itself then for the air conditioning required (this thing pumps out a lot of heat)
• Noise (of which there is a lot)
• IT support bills (both Windows Server and Exchange are pretty complicated, and there are many ways they can go wrong)

I've found ways to replace many of these functions with cheaper (better?) alternatives:

• Google Apps / GMail is a clear win for us: we have so many spam related problems it's not even funny, and Outlook is dog slow on our aging computers
• You can buy networked storage devices with built in print servers, such as the Netgear ReadyNAS™ RND4210 that would allow us to store/share all of our documents, and allow us to access printers over the network

The only thing that I can't figure out how to do away with is the authentication side of things - it seems to me that if we got rid of our server, you'd essentially have a bunch of independent PCs that had no shared pool of user accounts / no central administrator. Is that right? Does that matter? Am I missing any other good reasons to keep a central server?

Does anybody know of any good, cost-effective ways of achieving the same end but without the expensive central server?