I have seen the exact same with another MB (I do not remember which.. of all I've had.. think it was a ASUS board with ATI X200 express chipset).
About the same OC-limit with a Celeron D.. FSB192-194.. Could set FSB higher both in Bios and AI Booster.. without effect...
Sold the MB and bought another (i910).. and could OC way past FSB200.