To broadcast a 320x240 video, at roughly 24 fps, with decent video quality and sound. This is purely hypothetical, so lets say the audience consists of 1 user on a cable connection. Whats the target bit rate should I aim at? What should be the upload speed of the stream source?

That's hard to say since it will have a lot to do with the codec you are using
and the bitrate associated with it.

However, most people do not have a higher than 1 mbit upload speed (most
probably have 512 or lower) on ADSL (cable might be higher). 1 mb = 1024 kbit.

Divide the number by 10 (yes I know the correct division is by 8, but I use
10 to factor in overhead etc.) to get the KB/s (as in kiloBYTES) number.

So in theory a 1 mbit line should be able to do 102 KB/s up and a 512 kb line
can do 51 KB/s. Usually these numbers are lower (my 1 mbit upload here
usually stops around 80 - 85 KB/s).

Fortunately, almost all encoders rate have a datarate in kilobits and megabits
as well, so that's nice. For example, a DVD has a max of aroun 9.2 mbit/s
which you cannot real-time upload with a 1 mbit up line.

For your movie the datarate of 250 - 500 kbit/s is probably more than enough
and means you'll at least need a 512 kb upload line.

As usualy, do tests and divide the final file size by the duration (in seconds!!)
to see what the datarate is you need to support. An example.

If you have a 50 MB file that is lasting for 5 minutes (ie 300 seconds) you
need a line capabale of doing (50 * 1024) / 300 = 170.6 KB/s or 1706 kbit/s
(almost a 2 mbit upload link).

That as a very helpful post Rob, thank you.