mysql --> Commercial DBs, When will I need to upgrade?

I've been programming PHP w/ mysql for almost 2 years now for my company. We develop online testing and surveying software. We are currently running operations for a few specific companies where maybe 20-30 tests will be taken online per day (it requires pulling the test questions out of the database in random order, and then putting all of their answers into the database at the end of the test.) This is currently running on a shared webserver using php3 and mysql. Recently we've been talking about some projects that will require a lot more use. (Could be 100 people taking a test at once or it could be 1000 people taking a test at once. We don't know yet.) If anyone has had some experience with upgrading as your operations grow, I'd appreciate if you could answer ANY of the following questions: 1. At what point will mysql blow up (how many tests could be taken at once? How many rows of results could be stored in a table before it bogs down?) 2. At what point will we need a dedicated server instead of shared-hosting? 3. How fast of a server do we need? Will a 1ghz server outperform a 500mhz server when using apache-php-mysql? 4. If we need a new database, what is the next step above mysql? I have some experience with Oracle but it is too expensive. Is there anything inbetween that is friendly to PHP? 5. If MS-SQL is an option for a database-upgrade. What are the implications of switching our server to a win32-based server? Will we have problems with PHP on windows when all of our scripts were programmed for unix? I realize these are a lot of questions and that we probably need some consulting work done, but if any of you could share your knowledge on any one of those topics I would really appreciate it. I just need something to give me a head start in my research. Point me in the right direction! - Doug Schasteen dschast@escindex.com P.S. - if you know of any good articles online that compare different servers or databases please share.