Todd Peach , Nov 25, 2006; 12:20 p.m.
It always takes me a few minutes rattling around this other site to locate this page:
That's a pretty concise explanation (with photos) of the differences between PB4/5/6.
Starting with what you have, the 50mm is probably your best bellows lens.
You may need to use an extension tube as a spacer with your S2; there may be inadequate clearance to the bulging battery compartment.
For many folks, their first equipment purchase related to close up photography is a dedicated macro lens in the 90-105mm range. Many of these allow easy shooting down to the 'life size' range, where you are filling the frame with an object 24x36mm (on film) or 16x24mm (on a Nikon DSLR).