Is it possible to upgrade a Mac Pro (2007) with Lion (10.7) to Yosemite (10.10)?
OK, this might be somewhat opinion-based, however...

Possible, yes.
Worth it, no.

Possible
There are tools &amp; online guides far too long to repro or précis here on how to achieve it. It requires hacking the firmware &amp; needs re-hacking at each update -
These guides from MacRumors are as good as they get, one for Yosemite &amp; another for El Capitan.

Worth it?
The 1,1 &amp; 2,1 are really end-of-life &amp; now very slow compared to a 'modern' machine.
I really guess it depends on your budget, but you can get an absolute top-flight 5,1 [or the smart money goes on a 4,1 with the Apple upgrade kit] with two 6-core hyperthreading Xeons @3.46GHz &amp; 64GB RAM for under £1800 in the UK. Your existing drives will just carry over [though idk about the OS itself, mine did from a 3,1 to 5,1].
The Geekbench scores on those are about 32k, that's slightly under the current top model 'trash-can' at 33k. I just checked the score for the 2.66 Mac Pro, 6k.

I took the latter path, even though I was only moving up from a 3,1 - which is now in the hands of my partner &amp; very happily running El Capitan. I wanted a speed boost &amp; the upgraded 4,1 was right on the money for me.