I can't see the value in adding anything to the base Mini. Once you add an upgraded processor, you might as well just purchase the Mini Server.
For memory, getting the Apple 4GB upgrade is an especially bad deal since you will get 2X2GB. With this configuration you will have to replace the memory again in order to go any higher. You would be much better served buying the base Mini and adding a 4GB stick. This way if you ever want to go to 8GB, all you need to do is add a second stick.
I say go base Mini or Mini Server, I just cannot see the value of anything in between. I personally was going to go Mini Server, until I decided that I could benefit more from running the memory maxed. Having to replace the memory that came standard on the server defeated the extra value in coming with more memory. My next machine is going to be a Mini Server, primarily for the server license, and not because of any performance concerns.
As for the difference between 2.4 or 2.66, IMO, only a benchmark could tell the difference.