I have no "loose" tracks in my MP3 collection. CD Singles are still releases that have a proper "album" name, the name of the single. Every track should contain an album name as the indicator of its source. It's also how you'll be able to browse releases (by album) with your Sonos (or other database driven music system such as Slim Center).

iTunes will likely not contain cover art for everything in your collection, so you'll invariably have to do some manual labor at some point. By far the easiest and most complete site to use for that is RateYourMusic.com in my opinion. Mainly because it's easy to browse by artist and see a relatively complete discography, but also because they tend to have cover art for nearly everything in their database, even obscure material.

I think running a script to pull the images from iTunes as folder.jpg and then using Tag&Rename sounds like the best solution under Windows right now. Of course you could also try and find that script I found that does the embedding directly from iTunes.

If you have the option of using a Mac for a little while you might consider that. Bring all your music on HD over to this Mac, fire up iTunes and dump everything into it. With an iTunes account signed in, select all, right-click for the option to get album art, wait and when completed, run the AppleScript to embed the iTunes art. http://dougscripts.com/itunes/scripts/ss.php?sp=embedart

It's going to take a while to add a large collection to iTunes, so visiting the Apple Store to do this may not be a great idea. Oh, your disk will also have to be formatted FAT32 unless you install MacDrive on your Windows box to format it HFS+ (or unless you install a Writable NTFS driver on the Mac)