MacPro1,1 upgrade to 10.6
According to the tech specs, you should be able to use OS X Lion (10.7) with the following specs:


Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
2GB of memory
OS X v10.6.6 or later (v10.6.8 recommended)
7GB of available space


That would suggest to me that you would need to get onto 10.6 (Snow Leopard) + all updates and then could get to OSX Lion (10.7).

Mountain Lion+ Specifies an "early 2008 Mac Pro or newer", yours appears to be the iteration just before that.