The performance hit for using that 4th slot is only 2% - 3%, and that only applies if you're not actually using all of the available memory. If your system gets to the point that it's using all 24GB, then that 4th slot is going to help immensely.
Phillie - As far as I know, either of the examples you site should work, but you'll get far better performance if you stick with three of the same type, or if you need all four slots, four of the same type.
The sweet spots are 6GB (3x2GB), 12GB (3x4GB), and 24GB(3x8GB). If you use your machine like the average user, the best price/performance configuration right now is the 12GB. Paying for the 8GB sticks really only makes sense if you work with large files and use a lot of memory, or are pretty sure you will in the near future.