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I have heard of people using the forum with 50Mb Access databases, but at this size performance will depend on how good the web server is and what sort of load there is on it. I have heard of some web hosts shutting down sites when Access databases get up to 9Mb becuase of the amount of strain this would put on the web server, as Access is a flat file database, each time data is required from the database the entire file needs to be opened and loaded into memory.
If you think your site will be busy and have allot of posts and users then SQL Server would be much better at handling this and also perform better.