Middle-tier implementation

Question

We currently have a large multi-tier system written in Borland Delphi.  Win32 Delphi clients deployed on Citrix with COM+ middle-tier, also written in Delphi, containing the business logic running on multiple win2k3 servers.   The database access itself has been abstracted out to a 'Datalayer' COM+ component and highly tuned to our specific back-end database.  Data is transferred between clients and middle-tier as XML data packets.

When migrating to .NET what implementation would be best suited to develop the middle-tier - is COM+ still a valid future proof solution in .NET. 

The system has to be very reliable and resiliant with 24/7 uptime and failover, scaleable for a large user base (LAN only - no internet access), high transaction thoughput, manage distributed transactions across multiple databases.

We are currently in the very early evaluation stages of proposing a migration to  .NET so need to go through a series of multi-tier prototype models to identify the best architecture to support our needs.  I feel a lot  of R&D, reading and late nights coming on and would appreciate any advice from experienced .NET developers in this field.

Thanks