Do plenty of research before overclocking. Nokia will know because the EEPROM in the chip will state that for x-period of time it ran with a core frequency of, say, 800Mhz and not the stock 600Mhz. Nokia will know and they have the tools to find out.

In answer to all of your questions, I would have to ask that before you ask too many you gather all the research data that you can. Start in the overclocking thread, and then check the overclocking Wiki. Read, understand, digest and then make the decision which way to go. After that, bear in mind that either custom kernel you download will not allow the recently released fCamera to run properly, and they need to be uninstalled.

If you uninstall incorrectly, your phone will be bricked, requiring re-flashing. Do not go down this road unless you are comformtable with it.