For 4:3 I would go with 20-21" if you can find it. The largest size you will find is 21", but those are getting hard to find now. For tech info you will want to see Anandtech's LCD thread
here. Pay attention to the differences between VA/IPS/TN panels (described at the top).
Skip all the cheap <$200 panels if you want any kind of quality. Those are all TN panels, and most times they are widescreen. Rotated TN panels look like crap.
Things to check for quickly if you are looking at a particular model:
1) Vesa mount - panels that don't have a Vesa mount are TN. They are not meant to be rotated. A lot of cheap panels have come out recently that eliminate the Vesa mount, and instead use some POS built-in mount which provides at most tilt function (not even height adjustment).
2) DVI or HDMI connector - if all you see is RGB connectors then keep looking. The days of bad convergence and linearity need to just end already.
I was in the market recently to get another 20"-21" 4:3 panel and the lowest cost decent 4:3 panel I could find was the Dell 2007FP
here. It is either PVA or IPS (there is no way to know which one you will get). Personally I prefer PVA panels, but many like IPS better.
There used to be other good 4:3 panels - Samsung 213T, 214T (21") or the Viewsonic VP series (20"). I can't find sources for those anymore. Perhaps someone else knows.