How to upgrade my Mac Pro to an OS newer than 10.6.8? I have a Mac Pro running Snow Leopard. I tried upgrading to Mavericks, but the App Store won't let me download it. It directed me to this page. At first I thought it was because I was out of space (8GB required) so I deleted some things. Now I have 8.17GB available with 5GB of RAM (2GB minimum). The only thing I can think of is that my Mac Pro is arbitrarily too old. However, I don't know how to confirm that. I'm not even sure if there are any upgrade options available. What do I need to do?
It sounds like your Mac Pro is not supported (too old?) by a newer OS than 10.6.8.

Check by looking in the System Information app, to see what version of Mac Pro it is. Supported versions are "MacPro3,1" or greater. Unsupported versions are MacPro1,1 or MacPro2,1.

The reason why the older models are not supported is not arbitrary - it's because the first models had a 32-bit EFI, and the later ones have a 64-bit EFI. The later operating systems require a 64-bit EFI.