Mac OS Mountain Lion

The Mac Pro 3,1 introduced in 2008 can run Mountain Lion.

If yours is a Mac Pro 2,1 model, then NO.

That speed is a 2008. You can install Mountain Lion.

Anecdotes here indicate support for older graphics cards seems to leave a lot to be desired. Of all the older cards, only the 8800 seems to be well-supported, and it has a history of bad solder joint problems.

word of advice: do a clean install and keep Snow Leopard.

ML has a habit of showing powre management issues with the 3,1 - antedotes point "maybe they didn't test ML on the 2008" or something.

The 3,1 was first with EFI64, so it can, but maybe it really needs and will never get firmware update, and might have issues with its PCIe 2.0 as well that affects some controllers and GPU as well as booting issues with some 3rd party controllers.

Do dual boot.

The lack of Rosetta, the fact that many have a lot of old programs, extensions, background tasks ready to cause mischief. It will work better w/o those. "Less is more" esp with a workstation performing at its best. - Can even have too many "smart utilities" checking status of drives, sensors and the rest.