Do not know if this has been posted already, but the folks at the review site,
www.anandtech.com have compared the two versions of the Q6600 and you can find it
here.
just quoting the conclusion of the article linked above
"Power consumption of the new G0 cores is not only better than their predecessors, but it seems to scale better with clock speed. As we overclocked our Q6600s total system power consumption increased at a slightly lower rate with the new G0 stepping than with the previous B3 cores. The power savings aren't tremendous, we're talking on the order of 3 - 6% depending on clock speed, but it is a measurable improvement.
The impact on overclocking is a little less clear, at least with the quad-core Q6600. We were able to reach a higher maximum overclock with the SLACR stepping Q6600, but our highest stable overclock was not that much more than our original B3 stepping. If we look at percentages, our G0 part managed a 3 - 6% better overclock, putting overclocking potential in line with the power savings we noted. It's not a tremendously better overclocker but we suspect that on a whole it will yield tangible benefits.
There's no doubt that the new G0 stepping cores represent the latest improvements in the Core 2 lineup, but it's worthwhile to keep expectations in line with reality. If you're looking for a huge change in power consumption/overclocking headroom, you may have to wait for Intel's 45nm Penryn cores."
For me this means if you can get a G0 Q6600, good, but if you get the B3 your not really missing out that much. That is just my opinion based on the above conclusion.
Hope this information helps.
HD