MMPORG server side development guidelines

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.

That's very hard to answer. What you need to do depends on what you're going to do. The general, vague answers are that you should pay attention to efficiency and speed, but how you do that depends on the actual task. Don't expose method calls that can give a user too much privilege, but keep in mind that any web services you do make 'are' public.

MySql, MSSQL, I think they'd both suit the task well, though others may have a different opinion.

Well, what I'm really interested in is some reading about this. Would a server side development for, say, 50 users be generally the same as for 50,000+ or there are different principles involved.
And the question about the db engine has solved itself already. We're sticking with MS SQL.
Anyway, thanks. You didn't tell me anything I had not known already but that only confirmed my guess that nobody really would and there are probably no 'guidelines' or 'industry standards' about MMPORG development :)