Anything Intel adds an "X" to isn't worth the eXtra cost involved, even buying second-hand. Bus speed has almost no impact on performance (2-3% max in all testing I've ever seen). And anyway, like Larry said, just set the fsb to 333 (1333 bus speed in Intel marketing talk) and you've got the exact same chip sitting there.
If you're willing to spend $150 more than a Q6600 just get a Q9400/Q9550 instead. Those consume less energy, generate less heat, make your power bill lower and might cook you breakfast in the morning.
