What do I need to know about running my own dedicated server (with windows 2008)

You should be able to run Server 2008 on this ok, and you may be ok with Exchange 2003, although you may be pushing it a bit if you have alot of users. Exchange 2007, however, is not something I would run on this. Exchange 2007 needs alot of memory, it struggles with 2GB, with minimal users, I would reccomend at least 4GB if you have multiple users of Exchange, it also requires a 64bit CPU and a 64bit version of Windows.

As for administration guides, theres lots of resources on the internet, you may find some useful videos in the free videos section at CBT Nuggets.

Is this a server for you to play with and test things out on, or is it going to be a production server?

If the latter, I strongly suggest two servers. SQL and Exchange do not get along well together. They're both memory hogs, they can both use the disk heavily.

Please consider that this processor is a single-core, 64-bit-capable chip that cannot hardware-enabled virtualisation and may be limited by the amount of memory that can be added. I'm sure you'll be able to get the code list you mention running on the box but I wouldn't expect it to be particularly fast.