If you have a 2006 or 2007 Mac Pro then you can upgrade it only through Lion. It will not boot Mountain Lion. If Lion will work for your needs: You can purchase Lion by contacting Customer Service: Contacting Apple for support and service - this includes international calling numbers. The cost is $19.99 (as it was before) plus tax. It's a download.
If you would need Mountain Lion then you need new hardware which means you should sell what you have and get a newer model - preferably a 2009 or later Mac Pro if you prefer the tower, otherwise, you might consider an iMac instead. An i7 iMac benchmarks about the same or a tad better than your Mac Pro.