The Corei7's are generally faster in most situations. I wouldn't justify buying one of the more expensive Corei7's (940, or 965) over the cheaper one though (920). The performance boost isn't worth the extra few hundred dollars it costs.
You can't judge a processor based on just the number of cores, and the frequency (the ghz rating you mentioned). It seems like you are already knowledgeable about this though.
It really depends on which one you want to get. If you have a motherboard that will support a recent quad-core processor, but you have an older processor, it's probably more worthwhile to just get a quad-core. However, if you're building a new gaming or higher-end system, even though it may cost more, I would recommend getting a Corei7 system so you are more futureproof.
If you go to this link: http://www.tomshardware.com/charts/desktop...chmarks,31.html
, there are many benchmarks that compare most of the modern processors in different applications.
"I think there is a world market for maybe five computers." - Thomas Watson, Chairman of IBM, 1943