I suggest you read the U-boot thread (http://talk.maemo.org/showthread.php?t=81613) carefully. Take your time and ask questions (there, or here, as this is your thread anyway . Once you get the picture, I highly recommend that you install U-boot.

In general you only need one kernel. Kernel-power or CSSU-kernel (which are the same anyway) is sufficient (let's say it this way, stock kernel doesn't have any advantage over kernel-power, but kernel-power has many advantages over stock-kernel).

Even if you stick to one single kernel, it is very useful to have U-boot, especially if you tweak and mod here and there. The possibility of using the U-boot console to boot a specific kernel with very specific parameters has saved one of my N900s, which was unbootable and both SD card and USB didn't work!

Plus it will allow you to experiment with Nemo or any other alternative OS's without having to touch Maemo.