Wildbit , you could downsample via VLC but you'd start to get pretty poor quality I think. As a reference , I always use WMV1 at 768kb and a single audio channel at 96kb. I set scale to 0.75 and my mates are very happy with my streams.
With a 768 upload, you could try putting the video at 512 and the audio the same at 96, it doesn't leave you much left though.