Here are a few short things to keep in mind:
First, please get a good backup. I recommend SuperDuper! because it will give you a bootable copy. (Time Machine is pretty thorough, but it's not bootable.
Second, you can upgrade from Snow Leopard (10.6.8) or from Lion (10.7.4). Be sure your Mac is in the list of approved hardware.
Third, if you are running a Server version of OS X already, then you'll need the updated OS X Server 2.0 in place in order to upgrade. That is also $19.99 (link) if you haven't purchased it already.
Finally, when you start the upgrade, know that it will take 40-60 minutes. If the machine is sitting in front of you that's no problem. But, if you have a remote Mac mini server then that takes patience. Be sure to give it plenty of time before you start any remote reboots.
If you have any questions, feel free to catch us @macminicolo on twitter.