I’ve found with You Tube, that it’s not neccessarily that you want to have the largest filesize possible, but that you want to follow their specs of 320×240 and 29.97 fps. They do recompress their videos anyway, but as long as you stick to those specs and use a good mpeg 4 codec, like h.264, or I frequently use the Sorenson AVC codecs, you’ll get good results. Check out Current TV for a spec sheet, I’m sure they’ll have a set of specs to get optimum results. Every server will be different and have different bandwidth requirements.
To answer about is there a point to where bitrate won’t affect the video quality, yes there usually is a point to where higher bitrate doesn’t neccessarily result in any cleaner video, but that point will depend on the codec. I find with h.264 at 320×240 there’s rarely a need to go much higher than 768 kbs. Sometimes with very high motion video I’ll go up to 1200 kbs, I always spit out my first encode at 768 to get a good frame of reference.