What a 2.66 quad vs a 3ghz dual? The 3ghz dual will win due to higher clockspeed. Now most new software is multithreaded and a quad will run several applications better than a dual but just with a single program the dual will be faster due to clockspeed. Only a few programs can use 4 slower cores at once better than 2 faster cores. All in all it is better to have a quad because more and more programs are able to use the extra threads.
Are you comparing a Q6700 vs an E8400? My E8400 is still going strong but thats at a 4+ghz overclock.