The answer really depends on how much money you have to spend and how much time you wish to dedicate to the setup of your office systems. I'll outline a couple of configs below that I know will work together.
For a budget of approx $5000, you can buy four desktops, related software, a network switch, related wiring, and a multifunction printer/scanner/fax. One of the desktops should be a dedicated "server" as Adam describes above. I would rely on XP Pro for the Operating system for all four of the desktops given this low end configuration. The drawback of this config is the scanner - a flat bed scanner is just too slow when you pick up the volume. Here is an example shopping list I did at Newegg.com to help:
https://secure.newegg.com/NewVersion/Wishlist/PublicWishDetail.asp?WishListNumber=5833545&WishListTitle=Amazing+Charts+Setup+%2D+Low+End
If you step up to about $8500, you can buy a network attached storage unit (NAS) to dedicate to the database, four desktops, a laptop for yourself, network switch with wireless g/n capability, related wiring, a multifunction printer/scanner/fax and a high speed scanner for ID cards, labs or records from other practices. I recommend buying a uninterruptible power supply as well in case of power outage and connecting to the NAS and network switch in particular. Here is the example shopping list I did at Newegg.com for this level:
https://secure.newegg.com/NewVersion/Wishlist/PublicWishDetail.asp?WishListNumber=5832865&WishListTitle=Amazing+Charts+Setup+%2D+Top+Level
You could purchase a server with some redundant components instead of a NAS as Adam suggests but this decision really depends on how much time you wish to dedicate to maintaining the system. A NAS requires less time, configuration and space to get it to serve up the database. However, a server has more capabilities that you could leverage as it has a full operating system (such as email, web server, ftp, etc).
With either option above you should have an internet connection at the office, preferably a broadband cable or DSL connection. This will allow you to easily set up a regular backup to a remote location using one of the products mentioned elsewhere on the forum (backup services like mozypro, idrive or carbonite). Don't skimp on this one - be sure you backup the data for your practice remotely and on a regular schedule.
With the shopping lists above, there are certainly ways to save money vs. buying all new. You could use ebay, build a system yourself, or purchase different make/models, not buy so many MS Office products, etc. It comes down to how much time you want to spend on the IT side of the business. Also, you could pare down the number of desktops to only 3 - one for receptionist, one for single exam room, and the other for the database. Balance saving this money though with the time you would expend.
