As always, if you do the upgrade, consider making a full system backup beforehand. While I've done plenty of system software upgrades over the years without incident, my personal upgrade from Vista Business to Vista Ultimate was problematic.
A month ago, I upgraded my Vista Business desktop to Vista Ultimate. I used the same Vista SP1 DVD that I'd used for the original install. The Ultimate Key was one from the Server 2008 launch last May. You'd think this would be an easy upgrade. Mine took a couple of hours and reported zero errors.
The result wasn't quite stable. The original Vista Business install had been COMPLETELY stable since its April creation. After the Upgrade, the system froze once a day. It'd completely lose mouse and keyboard input and the desktop clock would quit advancing.
There was no memory dump, and no Event Log message to indicate WHY this was happening. The hard drive and memory both passed extended diagnostics tests. I updated every piece of software and every driver that I could think of. Then I removed every piece of (pre-existing) software and every driver that I thought might be causing this. No luck.
I fought it for a week, trying to find the cause, and finally gave up. I'd cloned the original Vista Business system onto an identical hard drive using Windows Home Server backups, so I put the original system back into place. It's been completely stable ever since. But if I hadn't made those backups, I would have had to re-install everything from scratch.