Thanks Paul for the explanations.
For example, if you type some keyword, and it's included in more than 80% of postings, search will fail saying the search term is not precise. FULLTEXT search is far from perfect in other aspects, too.
Ah, so that's the reason (I often encountered such behavior in search and now it explains why the search resulted in error message..).
I suppose it doesn't matter. Such kind of server should handle just your forums engine - displaying first page, topics, posts, user profiles etc. As about search, it's the most complicated part of it and mySQL simply has no power to handle it on millions of records.
That's good to know too not to pay unnecessary money for something it's not possible to achieve anyway..
But I hope in the future when this "million" problem becomes obvious, maybe there will be other solutions.
I see some custom search solutions (especially the paid ones) from Google and Yahoo, but it's not going to be perfect. Maybe the future implementations of MySql will bring something more useful indeed.
I've also read about some attempts to index the records in RAM memory somehow (so if database has 1 GB, it would be somehow cached to RAM memory and be instantly available), but that's probably something theoretical rather practical or possibel..