I am after some advice, I already have an i7 870 which is running at around 3.5ghz. It's using a P7P55D e Pro motherboard from Asus and has a single 7970.
I'm contemplating upgrading to a new CPU and board but most new chips are around the 3.5GHZ before OC'ing. Not a huge jump from where I am already.
Is it worth upgrading? Would it make any difference if I wanted to run two 7970's in future? I've seen pleanty of reviews comparing I7 2700k to the latest ivybridge chips but nothing comparing to older generations.
New architecture also bring better processing performance without the need to increase clockspeed. For example, the average performance increase going from a 1st gen Core i3/i5/i7 CPU to a 3rd gen CPU is about 19%. So a 3rd gen CPU running @ 3.5GHz is roughly equal to a 1st gen CPU running @ 4.15GHz. Actual performance varies with the actual program / game. Most games are not CPU bound so you are not going to see a 19% boost in performance. More like at worst 0% and at best probably 9%.
Generally speaking, Sandy Bridge CPUs are 12% more powerful than the 1st gen Core i3/i5/i7. Ivy Bridge is another 6% more powerful than Sandy Bridge.
It seems your current rig is fine. I would not bother upgrading until at least Haswell comes out. No one is sure about the performance increase over Ivy Bridge, but my guess is less than 10% because Intel is focusing on power consumption more than performance gains. Broadwell in 2014 will likely be a 10%+ performance boost over Haswell since the die process will shrink from 22nm to 14nm thus allowing Intel to increase performance while maintaining low power consumption.