I bought my pre-owned 2006 Mac Pro right before Mountain Lion launched not realizing that it would not be supported. It ran Final Cut Pro X, Motion and Compressor so it didn’t matter to me too much until rumors of the new FCPX 10.1 mentioned that it require Mac OS 10.9 Mavericks.
Thank goodness the fine folks at www.macrumors.com detailed a method to install Mavericks using a modified EFI from user Tiamo. I already have a ATI 5770 card installed, so it was a pretty smooth upgrade. It always helps to have another more recent Mac available to you.
The hack works beautifully and my 2006 Mac Pro has been running Mavericks for over a week now along with the new version of FCPX, Motion and Compressor. A slight caution is if and when system updates come along as we worry whether it will overwrite the EFI. (10.9.1 was a no big deal). Always follow along in the Internets for user reports.
A curious thing about my 2006 Mac Pro under Mavericks that while my external firewire iSight camera functions normally, it can interfere with boot issues.
The other great thing to do for the 2006 Mac Pro is the CPU processor swap. Swap the original processors with Xeon 5355 SLAEG processors and you will almost double rendering performance. I got a pair from eBay for $50. Make sure you get a long handle 3mm hex tool of at least 9″ in length.
Here are the links I found helpful if you want to attempt this upgrade.
There are also countless videos on Youtube that show the process. Good luck.
I have extended the life of my Mac Pro for at least a while until I can save or justify another Mac.