SQL Server is vital for any BizTalk environment, ensure maximum performance for the most demanding databases with a fast disk subsystem, consider using RAID 1+0 and SSDs for a high performance environment. In the majority of the BizTalk environments this is the first physical bottleneck you will face.
When doing load testing you will find that after you configure you environment to avoid any logical bottlenecks you will finally find a physical one. Normally the very first physical bottleneck you see is the IO disk subsystem performance on the SQL Server, this makes investing in the disk subsystem the most important scaling you can do on an environment. Betting on a highly available RAID system with fast disks is very important.
Read more on Monitoring and Reducing Database I/O Contention