Magic number appears to be 192: any greater than that and I get incompatible format errors.

64-112: Crappy video quality playing at full speed.
113-128: CPU is -almost- at 100% load. Probably ideal.
129-160: Nicer video, occasional slowdowns.
161-192: Best video/audio, frequent pauses and skips.

This was with Real Alternative on my home box, over a link that can sustain about 256kbit. I'll post again if being inside the network changes anything.