This is a processor thing, not an age thing.
A 1,1 Xserve is based on a totally different and incompatible architecture, though since it is very related to an early Mac Pro, it belongs there and not here. There are simply no questions relevant to both architectures (PowerPC / Early Intel) so this forum would be a mess.
Well, I'm quite sure a thread with over a 1M views regarding El Capitan on the Mac Pro 1,1/2,1 has a very good place in the Mac Pro forums and I have not noticed any "buy a new Mac" there. I believe it's a matter of attitude and asking the right questions in the right places.
That said - the OP should refer to any "early Mac Pro video adapter" thread he might find in the Mac Pro section. The answers are right there.