Hi Dennis,
Geoff beat me to it - the rollei close up publication is an excellent source of information. I'll add that I have used most of the equipment listed in there including the bellows and tubes that you asked about plus the reversing coupler and the M-componons. At a certain magnification and larger the bellows and tubes starts to get unwieldily. I've used a bellows plus the 68mm and 34mm and its getting a bit crazy to stabilize the set up but if you can move the subject instead of the camera to focus then its not too much a problem. If you have the 1.4X Longar that's a great way to get closer too.
The 60mm, 80mm and 120mm lenses work well reverse mounted and can achieve much higher magnification than one could get with tubes alone (you can still use the tubes in addition to the retro adapter). Beyond that the m39 shutter adapter mounted on the bellows with a M-componon or Leica photar optic can get you even closer up to 10 times magnification.
Regards,
Eric