I have the same setup, Mac PRO 1.1 with R9 280X graphics, 20gigs of RAM and SSD.
I am now playing fallout3 in full 4K resolution on my 40" screen and it rocks. Loading the game is almost instant. I cant read the text on the splash screens before next scene/map is loaded when i go into buildings or fast travel.
I now want to try falluot 4 and i am a bit curious on how you did. Did it work out well for you?
The 1.1 Mac PRO is 10 years old but still running. Nice machine, i understand why apple now quit making upgradable machines.
In fallout 3 the extra processor cores are making trouble for the game, and you have to disable them in the INI file otherwise you will suffer random crashes. Probably not the case with fallout4 but anyways.
//GF