Why I can't upgrade my Mac Pro OS X 10.7 to a more recent version?
As indicated here, the last officially supported system software is Mac OS X 10.7.5.

Any later system (10.8-10.10) is not running on your Mac and you won't be able to upgrade.

Here (english) or here (german) you'll find a hack/workaround to get Yosemite running on your MacPro1,1, if it doesn't contain a 7300GT or X1900XT video card (which is true in your case because your Mac contains a GeForce 8800 GT).