The BIOS we used for our testing was P1.10 which follows the typical BIOS layout that ASRock has followed (and also very similar to their parent, ASUS).
For most enthusiasts the place that you will spend the most time in is the OC Tweaker menu. Here you will find all of the settings to get the most out of your Clarkdale CPU.
Another place you will want to visit is the Advanced page. On this page you will find the settings for audio and video functions. Of particular importance is the Primary Graphics Adapter. As there is no auto setting, you will need to change this to PCI or PCIe if you choose to use an add-in card.
Overclocking the Clarkdale and the H55 chipset is going to be a little different than a normal board. As the Clarkdale has a GPU on the packaging, it can affect the stability of your overclock. Even if you do not touch the GPU, you will still have to bump up the voltage to the GPU a little to ensure that it does not give you any headaches.
With this in mind, we were able to get a very decent 4.3GHz out of the Core i5 661 with air cooling using the GMA HD for graphics, whilst adding in an AMD Radeon HD 5870 allowed us to hit 4.5GHz.
You can see the validation for the i5 661 with the GMA HD here while the i5 661 with the HD 5870 is here.
There is more headroom in the Core i5 661; it will take time to figure out where you can pull this from, but I am confident that 4.5GHz is not the limit on this CPU.
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.