I remember that here, on this forum, someone said that microstock business will start having technical problems because todays servers can't support extremely large databases as in microstock business. Is this already happening? Seem that most big agencies have some problems... 

I'm not sure who said that, but (if they said that) they don't know what they are talking about.

The problem with many of the sites out there is that they want to use free software (Linux, MySQL, etc).  Free software is fine for most stuff, but when a site gets large, then you should pay for the big guns.