Try it and see is the best way tbh but don't get too caught up trying to convince yourself that one sounds phenomenally better than another. Everyone's ears and h/w setups are slightly different and it all comes down to what sounds good to you and what you can put up with.
Just get a piece of music you know very well, rip it at different rates and then give them a listen on your h/w setup and preferably in the environment you'll most likely use it. Sat at home in a quiet room and you're going to notice the differences a lot more than say sat on a noisy bus.
The other thing is to work out how much music you need to fit in to how much storage. No point ripping at 96k if you have two tunes to fit into 60GB. Conversely, if you have 1000s of CDs to rip, anything to high and you'll run out of space.
edit: probably should add that personally I feel going to 160 or below is too low for me, VBR around the 192 or above range is where it starts to be good enough for me on my iPod a set of Shure's and getting me to work and back on the tube. But my main objective is to just cut out the noise of the tube in the morning and get me to work and back with out flipping out. It's more a background thing for me than a omg this music sounds awesome.