depending on the need
right now ISI game engine doesn't really support dual or even quad core...
it is still on single core
but most likely games coming out on other type of game engine will use more than a single core
if you are also in flight simulator, it is better to go dual or quad core is the best
if you are running video editing software, than quad core is better
Intel need to slow down. They are releasing Quad Core soon to the market, yet games and many applications don't support even Quad Core. Servers can really only take full advantage. Mind you, it's good to future proof, so Quad Core I say.
Was readin up on the i7 920, you can overlock to 4ghz very easily and you don't need liquid cooling (just a good heatsink). So you can save the $999 from the i7 965
Just remember, all the new and greatest hardware always comes out with little support. The hardware needs to get out there before the developers will use up the time and resources to take advantage of it.
I say quad core, because once they do start to take advantage of it, i bet the dual cores will get outgunned in short order.