the E2xxx line is the slower generation of the new processors - hence why it still carries the branding "Pentium", whereas the Qxxxx and E4xxx onwards line is the faster generation of the new processors, hence why they carry the branding "Core 2". The reason why I suspect the E2xxx is the same price is it was recently introduced(again, slower generation so it might enjoy some speed benefits by being more recent but probably just a clockspeed increase), whereas the Q6600 is from the beginning of this generation. Additionally, the Quad core has two more cores to boot and eight times the L2 cache(1MB vs 8MB) - while this won't make a massive difference, it's good when doing CPU intensive operations as more calculation routines can be fit in the chips memory, resulting in it not having to go back and forth from your system memory.
Unfortunately a quick google couldn't find any benchmarks for you to look at, which are also generally pretty skewed to quad cores since most benchmarking platforms can take advantage of all the cores. I find myself _regularly_ maxing out both my cores, but that's probably because I run my processor(E4300) at the stock 1.8GHz instead of 3.4GHz I used to run it at.
Additionally, a thing to note that as a generation becomes older, the manufacturer 'learns' how to produce faster chips with a higher success rate, and 'speedbins' them better(sorts them against their maximum clock speed more effectively) - this is why the E2xxx was probably introduced, as they could finally provide a large enough quantity which would be fully stable at 2.4GHz. Also, -don't- quote me on this but I believe the E2xxx chips are just normal chips which didn't pass the cache tests and thus have some disabled.
Disclaimer: I used to deal in building computers, etc, a few years ago, but lots can change so if you see something factually wrong, please feel free to correct me!