I just put my new system together with this board. It does support 166 FSB. I have my XP1700+ TBred B running on a 186mhz bus speed (default voltage).
Is your 2600+ a 133 or 166mhz BUS CPU? You have to set the jumper to its proper place. The CPU jumper block has 4 pins. Mine came set at 100mhz. With it set at 100mhz, you have the option of 100-132mhz in the BIOS. With it set at 133mhz, you have the option of 133-165mhz in the BIOS. With it set at 166, you have to option of 166-200mhz in the BIOS (all in 1mhz increments).
Double check it in your manual, but I believe the setting for 133mhz is no pins shorted & 166mhz is the bottom 2 pins shorted (I'm at work now & so I cannot look to verify it).