ok, I have a fx-8120 on 4.8GHz and a HD 7870
I just disabled 6 cores and downclocked it to 3GHz and took a look at RO2, I lost ~10 frames.
If I lower my coreclock on my HD7870 by 500MHz (from 1100), my performence decreases by ~35 frames.
I'll not even do a synthetic benchmark.
A 3GHz AMD Quad-Core is enough to play RO2/RS on ultra, and you can use it for other games too, because the majority already supports more than 4 threads and the trend will go on because the "next gen" consoles have 8 (true?) cores, no point in getting a dual core to save a few bucks.