Would it make sense for me to upgrade to:

OS X 10.8 should run just fine.

First, go to your Applications folder. For any downloaded application, right click and choose Get Info. Next to "Kind," if it says Application (Power PC), it won't run in Mountain Lion.

A few improvements are iCloud (although this was added in Lion), which is useful if you have other Apple devices, as it can keep photos, contacts and other content in sync across them. It also has Messages, allowing you to text any iPhone, iPod touch, iPad or Mac using iMessage. Those might be the two most notable features, though others may have a difference of opinion on that one.

A lot of folks ask this question when what they are really after is performance. There are far better ways to improve performance in a Mac Pro 65lb tower than Mac OS X software version upgrades.

What I generally say is that there should be features in the new version that you WANT. The new version will be somewhat different. It will have things you dislike. It may even have things you hate. So there needs to be a compelling reason to move from the version you know and are used to.