In the Advanced Options section of Preferences there is a setting for "Memory Usage" that can be set to "Low", "Medium" and "High". I have tried each setting and then watched the System Monitor for changes, but don't really see much difference between the three. (I am using Ubuntu Gutsy.) What do these settings do?

They control the maximum number chunk downloads per torrent. High can result in more chunks being downloaded at the same time then medium and low, and so more memory being used.

I say can here, because it all depends on the swarm, if there are only 2 peer to download from, we will only download 2 chunks at the same time.

How many is "high"/"low"? I have 2GB of ram, and I hardly ever use all of it (fork bombs are fun, though;).
Does it also affect uploading?