Heat differences don't just wiped off by your Gigabyte Windforce, it only get dispersed inside your case by the custom cooler(i.e making your other card in Crossfire a lot hotter and heating up other components)I. Regardless of the custom cooler used(unless it is a pure blower) your 7970 Ghz still generate a lot more heat than my 670.
As proof: My Gigabyte windforce 3x 670 is running 1150 Mhz with a 55% fan speed, but my temp is 51 instead of your 74 LMAO.
http://www.tomshardware.com/reviews/build-a-pc-overclock-benchmark,3276-12.html
This article showing a Gigabyte Windforce OCed to 1243 Mhz, staying at
52 C ,using the same custom cooler as your 7970 Heat Machine.
http://media.bestofmicro.com/W/O/348504/original/GPU OC 2.jpg
Also Power consumption makes a huge difference in picking the PSU, 2 7970 Ghz forces you to get a PSU that produces 150 extra W minimum since they consume 130W+ more than a pair for 670.
Facts are facts, the 7970/Ghzs run hotter, consume more power and only offer marginally better performance for significantly more in price. Unless you really want to game on a IPS panel(not recommended for FPS), they don't offer you anything a 670 can't do just as well for less.