Biztalk Topology

What would be the preferred topology required for a Biztalk Server 2006.
Information like 
  1. What would be Hardware Requirements.?
       i. RAM required
       ii. No of Processor Required
       iii. Speed of the Processor
       iv. Size of the HDD required.
  2. Whether its good to keep SQL Server and Biztalk Server in a Single 
machine 
      or  different machines.
  3.  Any other info related to this.

Thanks in advance