I've tried that. Not to be rude, but don't post unless you know the answer. I've been asking people all over the place and even EMailed ASUS, but I haven't recieved a yes or no answer.
The ASUS website says the BIOS upgrade is to make the mobo support 2600+. But if it requires 1.6V and my bios doesn't have that option, sounds like it won't work. Perhaps it will bring up the option when I install it, but I'm not going to spend almost $200CND just to test that option.
Yes to the 266 chips no to the 333 chip. With the new bios you will be able to set the temp to 1.65. If you cant for some reason, the cpu will just be more overclockable and you will need a better fan. Personally I never put the fastest chip on an old board. Seems too much, so I would recommend the xp2400.