Maybe the board only supports 100mhz fsb (400 quad pumped). I have an old mobo like that, it'll only take cpus with the 400 fsb (100mhz in bios). So if you get a 533(133 in bios), or 800fsb(200 in bios), then it'll underclock it to a 400fsb.
Two options in that case, get a new motherboard, or get a faster 400fsb cpu(they made 2.8ghz 400mhz fsb cpus).
Well, I guess there is a third cheaper option, to just settle for the 1.8ghz speed.