Finally some guidance on scalability! A new BizTalk Server 2004 download:

Scalability and High Availability in BizTalk Server 2004

The architectural components of BizTalk Server 2004 that facilitate scalability and high-availability are based on robust methodologies and mechanisms embedded and integrated within BizTalk Server, SQL Server and Windows Server 2003. This multi-faceted scalability and high-availability architecture provides numerous options for designing OSS/BSS integration applications that can be expanded to meet any performance and growth contingencies on an incremental or exponential basis without any operational disruption.