problem
The problem is that the board's max pci divisor is 1/4 and it kicks in at 133 FSB. So as you go higher in FSB the pci bus and agp bus continue to go out of spec. Some kt266/a boards can do 166 and even a little higher, but if you want those really high FSB speeds you need a motherboard with a 1/5 divisor whichi kicks in around 166 FSB... aka a KT333 motherboard.
Since you'vwe got a GB of memory, I'd say one of those sticks is holding you back. Try less memory and see if you can get higher on a single stick. Also try to adjust the memory timings and make them weaker... higher numbers for memory settings.