If all you use the PC for is home automation, I wouldn't spend the money to upgrade from XP. You would gain no new functionality over XP if you only use it for a home automation server. I've been thinking about making a small xp box like you are talking about just for home automation. It's a great idea. I believe I'd keep things simple and use XP SP1 if I ever build a standalone PC and I'd even use one of the new SSD drives for their fast read times.
The reason I started out using Premise in Vista was that I already run media center so that PC was already running 24/7. In addition, last year I was new to home automation and didn't want to spend a lot until I knew I would like it. That being said, my wife does complain when I want to stop/start the Premise service because I have to go the TV to do that. Everything else I do remotely using builder. Also, it's always better to stay with the OS the software was designed for. For example, if you use Premise on Vista or Windows 7, Rob's speach tool doesn't work; however, 123 posted a new version that does.