Well, not to brag, but it is awesome, lol.
OK, this question has been asked quite a lot, and there are a lot of threads on the subject. While it isn't quite as difficult to answer as who was the best rock band in the past 30 years, it is close.
There is no doubt you can run AC on a speedy computer. The only real difference is a server is faster in some of the motherboard components and is more redundant. The OS will allow you to set up a domain, something you can't do with a non-server OS. I wouldn't necessarily say that because you are using a non-server that you have to run peer-to-peer. A server OS comes with licenses, for instance SBS 2003 will come with five, giving you four for your users or computers (I would definitely do per PC). And the others could be covered by five more.
I personally always say go with a real server and set up a domain. I have my favorite analogy now. If your roof suddenly began leaking and water was falling in all your exam rooms and on your fax machines and copiers, etc., would you buy a new roof simply to protect AC? So, why does everyone look at their computer setup only for AC. Your network is for AC, your billing, your scheduler, your files, your email (with Exchange), your companyweb, intranet (Sharepoint) if you go with SBS and remote capabilities. Yes, you can remote into a regular computer, but a server with robust terminal services, etc. gives you a lot more secure options. Right now it may not be a big deal, but the ability to protect folders and files is very helpful. The ability to control who logs in to the server and when and passwords, etc. is great.
The question should always be do I need a network domain for my office, which will allow me to grow in a secure and organized fashion or do I want to go less expensive and go with a non-server workgroup? For me, half the fun, is working and tinkering with Small Business Server.
The VPN versus server and remote doesn't make sense as you can do remote without a server and VPN with. For most remoting, LogMeIn or RDP is fine. With SBS you can use Remote Web Workplace which is too awesome to describe. So is Sharepoint. Sharepoint itself will save your office hours a month and allow organization of your office that you simply can NOT do with any other software.
As for Vista Business vs XP Pro, that is a tough decision that someone asked awhile back. I use XP, but I am rather excited about Win 7. I would find it hard to purchase Vista Business and then having to switch due to Win 7's improved OS.
