You could try Spica TC-50s. They are inexpensive, light weight, and legendary in terms of sonics. Mounting them on a wall (preferably on a shelf) will not give the greatest imaging, but will improve their bass response. I have a pair that I wasn't using, and put them on the floor up against the wall (yes, smack in the corner of 1 wall and the floor)opposite the foot of my bed, and was astonished at how good they sound. While this setup provides only crude L-R imaging, there is a great sense of size and very natural, detailed sound. I remember hearing (I don't remember where) that some people mounted TC50s upside down at the junction of the wall and the ceiling with very good results. In terms of power, 20 watts should work if you don't expect too much in terms of volume. My first system consisted of the TC50s poweed by a Harmon Kardon HK380i reciever, which I think put out 30-35 watts/ channel, and I remember getting pretty loud in my small bedroom. They worked just fine with a 30 W Pass Aleph.
Small But Good Sounding?
I have a JVC desktop cd system that I use in my bedroom as an alarm clock. It came with some very small JVC speakers that were very thin sounding. I am looking for a pair of small speakers that will sound good mounted on the wall. Any suggestions? The JVC puts out about 20 watts per channel.