I agree with trying to skip Vista if you can. My company invested a ton of money and time with MS to get it to work in our 90K+ computer environment.

Based on costs, building your own PC's is not cost effective anymore. I'd stick with a standard, off-the-shelf HP desktops.

The 385 series of HP servers are pretty good and you may want to go with Server 2007 especially if you're looking at ever putting exchange on to them.

Create a base image with all of your software installed and configured and then just run sysprep and clone them using something like Ghost. Keep your support costs low by maintain your image (thus making disaster/recovery nothing more than a quick re-image). Don't allow users to store work on the PC... force the use of shared drives.

If licensing of software is an issue, create custom built install bundles for quick deployment using something like WinInstall or Ghost AI Builder.