Never heard of this uPnP Audio/Visual stuff, and really can't comment on Twonky, but this wiki article might be a good starting point for finding a decent software:
http://en.wikipedia.org/wiki/Comparison ... diaServersAs an alternative, too, bearing in mind I *really* don't know what this uPnP is capable of, I use Orb. Orb generates a web based interface, that you can log in to from anywhere via their website. Streaming is done via LAN if possible, and I've found it works quite nicely for my purposes (to my Wii). But it sounds like this uPnP is likely to be able to stream much better quality.