>even on the English Wikipedia (5 edits / second at most?) a single CPU would suffice

IIRC, the rate is far far higher than that. Perhaps ask a syadmin how many edits/sec (or are db transactions more relevant?) we’re looking at currently. 5 edit/s seems low to me. But maybe I have been dazzled by numbers such as “>80000 SQL queries per sec” (from http://dammit.lt/tech/velocity-wikipedia-domas-2008.pdf; probably all WMF, not enwiki) etc O.o

Mike