Looking at late '80s thru early '90s, the Trek 700, 800 and 900 series have cro-mo frames.

The 700, 720, 720, 800, 820, 830 have plain gage tubes.

The 750, 790, 850 and 900 series have double butted tubes - and are more desirable.

The 700 series have ETRTO 622 (aka 700c) wheels.

The 800 and 900 series have ETRTO 559 (aka 26" MTB) wheels.

I presently have in various stages of work the following: 720 (this one the brake tracks on the rims are too worm for me to feel comfortable), 750 (too small for me, be built in to a drop bar to lend a friend), 850, 930 and a 950.