multi-db and dedicated db server


Is multi-db necessary only when all are in one box (apache+mysql) or can it enhance the performance of a dedicated db server as well?

What is the deciding factor to go for a multi-db setup? Is it just the number of blogs or number of blogs+visitors+visitor activity? This is because I am planning to do an installation of wpmu with about 3000 blogs for a community. We do not expect much traffic / activity during the normal days. But they are expecting to have blog marathons for 2 days in every 3 months time.