Dual Core or Quad Core?

To get similar performance from a dual core i5 compared to a i7-760 you'd have to overclock it pretty aggressively, probably wiping out any thermal advantage gained.
However a dual core i5 could still perform well enough depending on what programs you're planning on running. I would take a look as various benchmarks to see if a dual core i5 is up to the task.

Using an integrated GPU in tandem with a discrete GPU is termed switchable graphics, and unfortunately I haven't heard of many desktop implementations available.

If you're concerned about heat buildup inside the case you could get a video card that features external exhaust. EVGA makes external exhaust versions of many of its cards, and many ATI (AMD rather) cards also feature external exhaust.

Edit: If you're not planning on playing many, if any, games, especially higher end games, you may want to look at a dual or quad core Sandy Bridge CPU on an H67 chipset motherboard. The IGP on Sandy Bridge is much better that the IGP on the i5-i3 Clarkdales, and Sandy Bridge processors perform better than a comparable Nahelem processor. However, this only applies if gaming is a very low priority, since the switchable graphics limitations mentioned about still apply and the Sandy Bridge IGP is too weak to play higher end games.