Gregory's ServerJust finished his biography by his PA, Rob Wilkins (Funny and sad and heartwarming and excellent and well worth the read).
That quote is in there, from a chapter in ~2007, but it looks to be something he said reasonably often to visitors so there might not be an original source?
@lewiscowles1986 @bison
"Why do you have six computer screens?" people invariably asked when confronted by this bristling display of hardware. 'Because I don't have room for eight,' Terry replies. And also because they give the Chapel a slight Mission Control vibe. And also because they look really cool with the green 'raining chipher code' screensaver from the Matrix flowing down all six of them at the same time.