whats most common naming convention for MP3s (what does iTunes store use?)

If you're using iTunes the way it was designed to be used (and certainly, not everyone does), you rarely to never interact with the files through the finder. Everything is through iTunes. And, as iTunes organizes by mp3 tags, the name of the actual file doesn't matter. So, as long as your tags are correct, you can use whatever file naming convention(s) you like.

Track#AlbumArtist, as you suggested in your original post sounds like it would work fine and allow you to search for files. Again, I do almost all my searching through iTunes (and my library is stored on an external). It seems to me that the only time it would really matter is if decided you didn't want to use iTunes any more and were moving to some other program which might care.

The last track from Joni Mitchell's album Shine is titled thus:

10 If.m4p

So, it appears that the iTunes Store just does track number and song title.

The iTunes Store sells both protected and unprotected aac files but not mp3. That, by the way, was the file name. The ID3 tags show artist, album, track number, artwork. Lyrics you have to add yourself.

I have no clue how to do a screen shot and post it. Sorry.

Both interfaces have their pros and cons. The 5th gen is my "workhorse". Mostly, it sits on my desk at work hooked up to speakers or in my iHome clock radio. The Nano, I use for running with a Nike+ Sport kit and Podrunner podcasts. The Touch is my choice for video.

I like MpFeaker for tagging but it's not free.

iTunes has no automatic tag look up function for anything other than CDs (through GraceNote). I import a lot of mp3s (legally obtained) that need tweaking. Also, GraceNote doesn't provide lyrics. If I just need to edit a couple of tracks, the iTunes interface is fine. But for bulk adding of artwork or searching for release dates or lyrics, I use MpFreaker. I find it works best if you do small batches and keep an eye on what it comes up with. Sometimes, well, rarely, it's way off base.

There may be better utilities out there but MpFreaker was recommended to me by someone I trust who spends a lot of time reviewing such things and it works for me so I haven't looked for anything else. You might find more reviews of other software at www.ilounge.com

Best of luck.