High Sierra on a Mac Pro 2008, technically the last officially supported version of macOS for the Early 2008 Mac Pro is El Capitan. However this does appear to be a software cutoff point rather than an issue with the hardware required to run it.
There are a large group of older Macs that have fallen foul of this cutoff (most of them being released in and around 2008), thankfully a rather enterprising Mac Rumors user going by the name of dosdude1 has written a patching tool to bypass this limitation.
High Sierra On A Mac Pro 2008 – Patching Thoughts
I can attest to the patch install running without incident on an Early 2008 Mac Pro (3,1) and being relatively easy (ignoring any non Mac video card issues), 90% of it being the same as doing an in place upgrade using a usb stick.
However as you are going to be booting from a USB installer if you are running a Mac with a non Apple video card you will have to do this blind (as my Mac Pro 2008 rebuild article mentions regarding booting into recovery).
I did find this part rather tricky to the point where I went hunting again (in two different houses) for my original Mac Pro graphics card. In a flash of inspiration I checked an old server in my father’s loft and low and behold there was the card, a quick swap later and I had my Nvidia 8800 GT back and ready to go! Now if only I had remembered where it was when I was rebuilding the Mac Pro, that would have saved me a lot of time and made a fresh install easier.
High Sierra On A Mac Pro 2008 – How To
I am not going to rehash the simple and clear guide that dosdude1 has written for building the patch installer as it would just be plagiarizing their work, you can find the guide here.
If you are looking for support for any troubleshooting or questions regarding the patching process or install a good place to start would be the Macrumors forum post dedicated to this topic, you can find it here.
If I can be of any help or if you have any question regarding this, feel free to drop a comment under this post.