My 2008 (3,1) Mac Pro runs Sierra just fine! Although I did add an SSD and 16 gigs of RAM. "GeekBench" of 12,500 or so, similar to some 2017 Win 10 boxes with an i7 (Intel Core i7-4960HQ). Best $800.00 I ever spent! (Got it "used" 2 years ago from a navy sailor/Hornet mechanic, got a discount because it had a dent in the upper aluminum handle).
Nice! I’m kinda tired of the heavy frame of Pro 09 and will probably buy on sale an Air that’s a year old.