As far as I know, there are no official configurations of this model that shipped with 3.06 GHz processor speeds. Since this was the last 20-inch model, it is unlikely you will be able to find a logic board with a 3.06 GHz processor already installed that is designed to fit in this machine. In other words, you would need to research upgrading the processor itself, not the logic board. Please check [https://everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-aluminum-processor-upgrade-instructions.html|this link] for more information. [br]
If you are looking to improve performance speeds and you have already upgraded the RAM, it may help to look into installing an SSD (solid state drive). If you have an HDD (older style mechanical drive) this may be contributing to slowness. This is also a much easier upgrade option than logic board replacement. See [guide|919|this guide] for a detailed description of the installation process (you may need a 2.5-inch to 3.5-inch hard drive adapter bracket to mount the SSD internally depending on the style that you purchase).
Hope this helps!