If your db is HUGE then maybe - but Access/Jet isn't the evil thing some people say it is. I do prefer MSSQL, but for most web apps and most forums, Access/JET itself won't be the weak link.
Another question worth considering is: how long has it been since you archived some of your old topics? If you haven't done it before or in an appropriate time frame for your forum, archiving old topics can potentially give you a bit of a performance increase.