If you notice in the BIOS of the A7A266, 12.0 is the maximum you can set the multiplier to yourself, with 12.5 being reserved for auto. If you want to use a 2400+, you need to set it to "12.5x (or higher)", or try auto instead of manual. I believe I had problems when I set a 2000+ in the same board to manual.
The way i understand it is that only Rev. 1.1 will take cpus like 2400+ and 2600+. (read it on other forum) I purchased a CPU 2600+ and with my rev.1.04 its only posting as XP1700+ @ ~1660Mhz. That really sux cuz i didn't know about it till after i bought the cpu. I upgraded the bios to 1012 from 1011 hoping it would give me higher factor then 12.5 but no luck, still showing only 12.5 max. I tried setting jumpers and same thing. Would be interested to know if anyone having other rev. then 1.1 was able to get 2600+ cpu going?