Search the web for Doug's iTunes Applescripts.
The ones I use are
"CDDB Safari" which will search the Gracenote CD database for track info.
"CDDB Tracks to iTunes via Safari" which will transfer the info found by the above to the iTunes songs.
"Find album artwork with Google" which will do a Google, rather than Amazon, search for images. (This script is actually how I found the discogs, since it will return image hits from there, as well.)
Still an awful lot of manual work involved, but I don't know of any better way.