The current versions of Windows and Linux run perfectly well on almost all decade-old hardware.
Specs and performance aren't the reasons Apple left these systems behind. My guess is that it's because of the transitionary nature, and the development effort involved in maintaining for both 32- and 64- bit EFI. Of the systems with that type of hardware, an extreme minority were the type that are going to run current Mac OS X releases very well. That's to say -- Mac Pros are fast enough. MacBook Pros, iMacs, and Minis from 2006 really aren't.
It can be hackintoshed as well.
But, this hardware is also so old that it's easily outperformed by even slightly older quad-core laptops and probably things like the 2011 quad-core Mac mini (which itself is really not as much faster than the 2014 dual-core Mac minis as people would have you believe, which are themselves now a generation behind the current MacBook Air.)
A Mac Pro 1,1 would make a great Windows/Linux box if you happened to have one, but I would probably not recommend someone go buy one, if only because the firmware will make Linux and Windows harder to do than is strictly necessary.