We received the new 48GB RAM set we ordered from Other World Computing for the Apple Xserve (roadrunner) that I installed El Capitan (OS X 10.11.5) on two weeks ago.
I installed it and this computer (which was plenty quick before) is extremely responsive now!
Below are some pics from the installation:
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_RAM_48GB.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_xserve_cluster.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_xserve_clip.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_xserve_inside_old_RAM_01.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_xserve_inside_new_RAM_01.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_xserve_inside_new_RAM_02.jpg)
(http://eagle.fish.washington.edu/Arabidopsis/Pics/20160609_roadrunner_screen.jpg)