Sorry, but in my book "serious business site" and "using Windows" don't belong in the same book. I'd recommend getting and learning BSD or Linux. If you're gonna go with Windows, you'll have to ask someone else, as I haven't tried using it for anything but specialty servers for about 4 years now. Sorry I can't be much more help.
A single PII 300 with 256 Megs of ram could run everything from sendmail to dns to apache/postgresql/php and any other things you want in Unix. In Windows you'll need one machine for DNS, one for a database, one for email, one for IIS/PHP etc... if you want to handle load well.