Changing the setting for the bitrate will only affect any new songs that you rip from CD's. The birtate of the any song thats already been ripped is not affected by this.
When you transfer your songs to the iPod, it transfers at whatever bitrate it is, there is no scaling down or anything like that done.
I prefer to use MP3 because if I ever switch from the IPod to something different, I know that the MP3 format is pretty much universal so I won't have to worry about all my music not working on a different media player.
-White 5G 30GB iPod Video w/ DLO Silicone Jacket & RadTech Silicone Dock Protector
-White 5.5G 80GB iPod Video w/ Power Support Silicone Jacket, RadTech Silicone Dock Protector & InvisibleShield Full Body Cover
-White 1G 512MB iPod Shuffle