Mind telling me how you've overclocked your Q6600?
I bumped the FSB from 266 to 333 on mine, booted into Linux fine, ran mprime (Linux version of Prime95) with no issues. I could see from looking at /proc/cpuinfo that when idle the CPU frequency was ~1997Mhz going up to 2997Mhz when loaded.
All I changed was the CPU FSB from 266 to 333 - is that all I need to do?
This thread has grown a bit since I last popped in.
Hope you are enjoying your new Shuttle.
On my SD37P2 I upped the FSB and underclocked the memory so that the memory was running at just above stock speeds. I needed to up the voltage to the CPU a bit in order to get it stable in Orthos etc for a few hours without returning errors. I'm not familiar with the settings in the newer Shuttles, have they any more adjustments?
Hmmm, shout if you have a SK41G to rehouse.....