I did the same thing, had a 300A@450 then upgraded to a 566@875
yes you should do a flash of your motherboard BIOS
it is really easy if you can follow some simple steps
I had to flash mine to get the correct multiplier and because of the size of my hard drive
It is always good to update your BIOS for upgrades
edit: make sure that you set "speed error hold" to the null value (meaning it wont be used)
otherwise you will get errors everytime that you boot saying the chip is unworkable and it will set your speed to 266MHz and you will have to reset everything each time you boot.