The Core i5 661 Clarkdale is an interesting CPU. As it has the GPU right on the package, we were concerned that we would see overheating problems with the IGP once we really pushed the CPU.
While we did not see heat related issues, we did see what seemed to be a voltage issue when we pushed past 4.3GHz. At this speed the IGP seemed to be unable to deal with the speeds and the ASRock H55 board did not have a high enough IGP voltage tweak to remove this.
In the end we were able to only get a 4.3GHz stable OC out of our i5 661. This may change with BIOS updates or on a different motherboard, but for now it was as far as we could go using the IGP.
Using the IGP
Using discrete graphics
You can see the validation for the i5 661 using the IGP here and the one with an add-in board here.
As all overclocking results are dependent on the hardware you use, your results may vary.
Results of our overclocking tests are included in the performance section with the stock scores.
Important Editor Note: Our maximum overclocking result is the best result we managed in our limited time of testing the motherboard. Due to time constraints we weren't able to tweak the motherboard to the absolute maximum and find the highest possible FSB, as this could take days to find properly. We do however spend at least a few hours overclocking every motherboard to try and find the highest possible overclock in that time frame. You may or may not be able to overclock higher if you spend more time tweaking or as new BIOS updates are released. "Burn-in" time might also come into play if you believe in that.