Most corporate environments do not use *nix based workstations and desktops. You may find that you come across web servers, email servers, collaboration servers and the like that are *nix based, but if you are looking towards desktop/computer support, odds are you won't be seeing these from an admin perspective.
MYSQL doesn't really have anything to do with "computer" support. It would fall under application support, web development, or similar roles. Its similar, but not quite the same as Microsoft SQL Server 2005. Just a database engine with query tools.
"Best" OS's is a broad topic. I'd try and have in-depth knowledge of Windows 9x, NT, 2000, XP and Vista for workstation/desktop OS's, and try to know 2000, 2003, and 2008 for windows based server OS's. I also know NT4, but these are becoming few and far between.
Can't hurt to have a little Mac OS X expereince, but only if you feel you may come across it within whatever environment you will be working.
EDIT: What kind of job/environment are you looking at getting into?
EDIT2: I'd also get to grips with Microsoft Exchange Server 2003/2008.
