The Intel 975xbx R.303 (I think) did not support the C2D family without a hard mod. I think there is a huge thread by FCG about how to mod the R.303 to run the C2d. Same thing goes for penryn. Had to be modded to work.
Edit: here is the FCG thread http://www.xtremesystems.org/forums/...ght=975xbx+mod
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