Jen,
Geoff has summed it up PERFECTLY. To add a couple of points, a true server usually runs server OS software. You have to understand that a real server has distinct advantages over non-server hardware, the ability to run a RAID controller for one. It also has redundancy of its own and has much faster throughput.
A server runs OS software that Geoff mentioned above. Clients run client software such as 2000, XP and Vista. You state that you aren't sure if XP will run with 2003, etc. XP Pro is MADE to run with server OSs. That is the whole idea. The others are too.
As Geoff says, do not worry about Microsoft support, etc. 2003 will be here for awhile. And, neither Geoff nor I understand the need to put the billing software (even if it is old) on external hardware. External drives are ususally for backups or you can use NAS for extra functionality.
Personally, I would get rid of everything on the clients and install either XP Pro or Vista Business. Enterprise and Ultimate are more than you need. Ultimate just gives you extra medial stuff you don't want your staff using.
I don't want to leave you with the impression that you have to have a certified IT person. I am not certified, Geoff is not either and either of us could set you up.
But, and again don't take this personally, given your questions, it is clear you need someone who knows networking AT your site. If you wish to use Exchange Server (comes with SBS) or extra on just 2003, you will need IT. To set up Active Directory and permissions, you will need IT. This doesn't even mention your Internet connection, firewall, switch, etc.
Again, as Geoff states, EVERY SINGLE THING that needs to be accessed by all NEEDS to be or SHOULD be on the server whether it's a pseudoserver or peer-to-peer. But, the advantages of Active Director are numerous.
One thing that always gets lost in the shuffle on here and I think Geoff would agree is that everyone talks about P2P vs Server/Client DOMAIN in relation to AC only. A true office is looking at many things other than AC that will have extra functionality using Active Directory.
Finally, the functions that AD allows you to do, such as permissions, etc. and also Group Policy are extremely powerful attributes of a domain. Users on here constantly talk about remoting in. Many answers are given. LogMeIn, GoToPC, Remote Desktop, but with Remote Web Workplace (with SBS only), you get the safest login that every user can use. Even that can be restricted via the server.
I would not try to set up your network with the help of a few of us on here. You really need someone there in front of you who understands your needs and can give you sound advice. Once you trust them, let them do their job and you can take care of the patients. Of course, watch what he or she is doing. Preferable this person would be available when you need him/her and could remote in when necessary to the server to fix small problems.
Geoff's answer rocked! OK, Geoff you owe me a six pack or something after all those praises.
