IMO, it depends on what you want to do with it... as said in numerous posts (including the above 3

) if you don't do video/audio processing or a multi tasking maniac then there is no reason to buy a quad core, especially since the new Core i7's are out (though they're expensive to upgrade to) and have a different socket, so upgrading would be a no-no
if I were you I would go for the E8400
if I were to buy me a CPU today than I would buy a C2Q (probably a 45nm version) since I already have an E6600 @ 3.2GHz
price wise, you can get a Q6600 for ~$189 and an E8400 for ~$165. By picking the E8400 you would end up with a (generally) faster, cooler, cheaper CPU, with a higher FSB than the Q6600, almost as much cache (6MB vs 8MB) and better OC potential.
sincerely, I don't find comparing a new high end dual core 45nm CPU with one of the oldest quad core 65nm CPUs (that now stands at the lowest end of the C2Q table) as a fair thing
people should be comparing the 8400 with the new Q9450... sure it costs twice as much as a E8400, but that's what you're essentially getting, 4 cores instead of 2.
also, it's possible to OC the Q6600 past 3GHz (where it would definitely leave the E8400 in the dust), but not unless you have great cooling, a good mobo and have a lot of knowledge about OCing
just my 2 cents...
PS: I voted for the Q6600