The D975XBX can definitely be overclocked; I've done numerous overclocks on it. In the OS, you could install IDCC 2.1 (Intel Desktop Control Center) and enable burn in mode. Once overclocked, you need to reboot for the effect to take place. That should allow you to overclock.

As a matter of fact, if you have AA# 304 and above, you should be able to run Core2 Duo including the Extreme Editions. It's also the only chipset that can allow upwards multiplier setting for the EE whilst D965 based boards can only have downward multipliers on EE Core2 Duo.