there is a utility that will let you flash a new bios using the system memory (meaning its not permanent) this will give you a chance to make sure the flashing will work. but of course now i cant remember the name of it. but i got it at guru3d.com.
I did flash mine but i wasnt worried about messing it up. I knew i was getting my 6600gt anyway.
anyway like the others said. unless you are having major trouble with your card then dont mess with it. it is only a 20mhz gain.