can I update from a 10.6.8 to a 10.7 on my MacPro?

Open the Mac App Store and try downloading Yosemite. If you get told it's incompatible, choose About this Mac from the Apple menu, check if the computer has at least a Xeon or Core 2 Duo(not Core Duo) CPU and 2GB of RAM, and if it does, click here and order a download code for Lion 10.7.

Back up your data and check your applications for compatibility before upgrading. In particular, Mac OS X 10.7 and newer don't support PowerPC programs such as versions of Microsoft Office prior to 2008.

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If it is a Mac Pro they all run Lion and can be updated from 10.6.8

I would install Lion on a second drive rather than upgrading your existing system drive. Keep in mind that some of your applications may not work with Lion, since Lion has no Rosetta support.