YES, there is a difference in temperature. A few degrees celsius hotter with the i7 CPU during max load. However, if necessary you can disable the turbo boost function (typically during gaming where both the CPU and GPU become very hot). I don't think you'll get a clear answer as far as noise goes i5 vs i7, as noone here has compared noise levels in both iMac models. What I can say that in OSX, you'll have to push the CPU pretty hard for the fan to rev up - I'm using the 3.4ghz i7 CPU.
From what I've understood, i7 will probably give you more smooth/instant reaction when doing very intense multitasking, but otherwise the difference is only noticable when doing decoding and other similar tasks. As far as gaming, remember that anything other than 1440p will look like crap, and 1440p is a very demanding resolution for a gfx card. Because of this fact, the bottleneck will be the 680MX, not the CPU. There are very few occations at present time where hyperthreading actually makes a difference in games. Maybe with games designed for the next console generation, noone knows...