You really must take a look at M$ Small Business Server 2003.
www.microsoft.com/sbs.
It's based on Win 2003 server, and runs on a single server. It is intended to support up to 75 clients. The Standard version includes Exchange, with the Premium version adding ISA server and SQL server. As far as I can tell these are the full products, just intended to be configured out of the box for the single server platform and integrated to the one system. You can't put components on different servers. Only if there is a likelihood of your exceeding the 75 client threshold might it not be economic due to the upgrade path costs to full separate M$ products.
I've been running SBS2000 now for 3 years with 20 users on a single Dell PIII 1000GHz 512MB RAM server, and it's done fine, although we don't use SQL or the web server function. However there are also some fairly meaty Access based databases running most of the day for bespoke company applications, so at times demand is quite high. Additionally the server has Antivirus scanning all incoming mail to exchange and ISA, as well as puching the A/V to the workstations. WOrkstations Ideally should be Win 2000 Pro or Wind XP Pro (Win XP Home will not work with SBS).
I suspect with the web server bit it depends on your intended use and traffic whether you might want that on a separate box, and do acknowledge the security risk of running your own server. Ideally it would be nice to keep that separate from your LAN.