Clock for clock the duo cores are FASTER than a quad in everything unless the software/program can take advantage of said cores; With that said there is very little that does so you gain very little by going to a quad core.
your absolutely right, you do gain "little" but you DO gain, you gain nothing at all except a 20 dollar bill by going with a dual core. Even you cant argue that they both perform the same while gaming in any resolution than bare minimum.
Truth enaberif, the e8400 that is sitting im my lesser system was bought because of your arguments. I bought that processor because the system i was building was exclusivly a gaming system i overclocked the hell out of it and went in to play and wasnt overly impressed. of couse the computer was better than the last because of the quad SLI, but really, after swapping out the dual for the quad i noticed NO benifit to my computers gaming ability. EXCEPT of course for the two games that i use that DO utilize quad cores, wich of course the quad was wonderfully benificial.
now without saying anything about the future, there arent many places where the quad will outperform the dual, but whenever you find those places, you'll be happy you had it, also while using cpu affinity your computer will be alot more powerful when doing any muti tasking.
My argument has never been that the quad is "better" or "faster" (because life isnt clock for clock and real life use says that the dual and the quad perform pretty much exactly the same) its always been "its a whole 20 dollars more, and will be awsome on the occasions you need it" where if you cheep out on the 20 bucks, you'll never have the option to use the horsepower.
so my (damn good) advice is:
dual @ 4 ghz vs quad @ 3 ghz = EXACT same gaming performance
dual @ 3 ghz vs quad @ 2.4 ghz = dual is greater than quad.
cheep aftermarket heatsink will easily overclock a Q6600 to 3 ghz.
if you plan to O.C. go with the quad.
if you DONT plan to O.C. go with the DUAL. (as long as the dual is greater than 2.4ghz)