Try attacking it from the other angle. Look at streaming services that cater to the customer you want, and have significant volume (maybe youtube) and see what they're pushing. You'll find there'a pretty direct correlation between alexa rating (popularity) and quality(minimum bitrate required). Vimeo will always have fewer users than Youtube because the user experience is poor for low bitrate users.
There are many other factors, and this should only form one small facet of your bandwidth decision, but it's a useful comparison to make.
Keep in mind, however, that you want to degrade gracefully. As more and more sites come online you'll start bumping into ISPs that limit total transfer, and being able to tell your customers how much of their bandwidth your site is consuming is useful, as well as proclaiming that you are a low bandwidth site.
Further, more and more users are using portable cellular connections (iPhone) where limited bandwidth is a big deal. AT&T has oversold many markets so being able to get useful video through a tiny link will enable you to capture market that vimeo and Hulu cannot.
Quite frankly, though, the best thing to do is degrade on the fly gracefully. Measure the bandwidth of the connection continuously and adjust bandwidth as needed for a smooth playback experience with good audio. Then you can take all users across the gamut...