With moderate volume, the best practice recommendation is to have an ArcGIS Server that is dedicated for GeoEvent Processing and another (or cluster of) ArcGIS Server that is dedicated to hosting Feature Services, Map Services, GP Services. Doing this helps isolate stream processing on GeoEvent Processor from other GIS services and allows GeoEvent Processor to have full access to resources on its isolated instance. The role of the EGDB (Sql Server) would be to back your Feature Services for persistence.
Does this information help?
Adam Mollenkopf
ArcGIS GeoEvent Processor for Server - Product Lead