Q6600 and Q6700 are FSB 1066. Both have 8MB L2 cache.
Q9300 and Q9400 are FSB 1333. Both have 6MB L2 cache.
A Q6x00 would outperform a Q9x00 at the same clock. Intel had to bring out the Q9x50 to beat the Q6x00 series.
The Q9xxx is not compatible with older s775 boards that only do FSB 1066. If you have an older core2quad board then there is no upgrade path. And that's the point. Intel no longer provides LOYAL CUSTOMERS with an upgrade path. The only option is to get a whole new system, ie new chipset and new CPU. Whereas in the past, Intel made sure there WAS an upgrade path; they released better CPUs with higher clocks, SSE enhancements, HT, bigger L2 or L3 cache AT THE SAME FSB. And remember the whole pentium OVERDRIVE series?