A while ago I upgraded my trusted old MacPro1,1 to Yosemite following these instructions.
I also purchased a PC version NVIDIA GeForce GT 640 2048 MB for $99 at Fry's, and a little later upgraded my memory to 14GB and got a Samsung SSD 840 EVO 250GB.
BEST. PURCHASE. EVER.
My trusted old box is humming along like a champ.