I am using the Rio for about 2 months now. I have finally converted my CD collection (about 500) to Hardrive. I admit I have used a rate of 128 except for classical music where I used 160. I would really advise to take a CD you consider to be critical in audio quality and rip it at different rates. Then compare them by listening to them over your Rio. I would almost bet that you won't be able to hear any difference beyond 160. There's really no point in encoding more information than your ears can resolve, even if space is getting cheaper and cheaper. Especially not if you plan to keep your CD's anyway so you could always go back and re-rip them in a few years if a new technology comes out (like a software update for your ears or so...)
I use the MP3's not just to listen to them on the Rio but also to share with friends, per email, on portable devices etc.... The smaller the files, the easier it is in those cases. When you comile a MP3-CD for example, you want to get a lot of files on there.
Just my 10 cents.