You usually only have to flash to the latest BIOS.. It's OK to skip some.
Generally, you want to flash your BIOS as infrequently as possible to minimize the chances of anything going wrong because if something goes wrong in a BIOS flash it can make your board stop booting.
Since the multiplier is locked, the only way you could overclock is to up the FSB. RobRich wrote a book all about overclocking.. I'm sure you could find a copy somewhere online if you were looking for instructions in book form