I saw many times people putting in their PC's, memories that according to the mobo specs, are not supported... Like, people putting PC 8500 memories while the mobo only supports PC6200... Do I have to make any special configuration in the mobo bios to do this?

Also, people would do that so if they were going to upgrade their mobo to one that can support the faster memory, they wouldn't need to go out and buy more. Or at least I would... shrug

So why the mobo manufacturers tell that the mobo supports a slower memory than it actually does??

It doesn't actually support the higher memory, but that doesn't mean it can't be forced to run at it's rated speed. Say a board supports DDR2-800. You install a set of DDR2-1066. That set of 1066 will slow down to 800 when you install it. The reason people get the 1066 is to achieve higher overclocks. Usually the RAM is the limiting factor in how high of an overclock can be achieved.

http://forumz.tomshardware.com/har [...] 93383.html

I just posted some questions that somewhat relate to this topic.

But you're talking about the processor and FSB overclocking, right?