Hello, recently I've been approached by a man who asked me whether my team would agree to develop a server-side for an MMPORG. His specifications require that the system could provide stable service for up to 50,000+ clients.

Honestly, I've no experience in developing such large-scale applications and I'm not sure I'm aware of all the 'hidden reefs' so I ask you whether a team of 1 database architect and 3 programmers can manage such a project and what are the general rules for such architecture. Which programming practices should be avoided and which should be implemented. I'm mostly interested in Fail-safety and security.

And one additional question regarding DB Engine. The primary decision to make is whether it would be MS SQL Server or MySQL. I'm inclined towards MySQL since it's free but since I'm not the one who pays I'm only interested in the advantages MS SQL server provides compared to MySQL.

Thank you very much for your input.