The specs you should look for depend on what you want to use it for.
I usually tend to look for flat screen trinitrons (sony) and diamontrons (mitsubishi, nec)...recently I just picked up a brand new in box 22" samsung CRT with a .24mm dot pitch and it does up to 2048x1536...It's not the best one out there, but it does produce a very high quality image....I think Samsung made their own tubes for the most part, but I'm not sure of this...can anyone confirm this?
I usually look for the smallest dot pitch (at least 0.25mm and lower) and large screen size (at least 19"-22") and capability of high resolution (up to 2048x1536). The reason for this is that I prefer to use my CRT as an all-in-one solution for all my retro computing from XT DOS all the way up to Core2Duo WinXP stuff. I do a lot of photo editing with my more modern Core2Duo system and don't miss 16:9 widescreen ratio at all for movies, etc....that's what my 40" SONY LCD TV is for (connected to the same system).
For strictly older DOS gaming (320x200 and 640x480) you don't necessarily need a large high res monitor, but I still use my 21" trinitrons for older gaming as well...like I said, it depends on your personal preference.
The only information I can provide you with regarding what tubes are found in some monitors is as follows:
- SONY Trinitron - Some Dell, IBM, and Sun Microsystem branded CRT's.
- Diamontron - NEC, Mitsubishi, and I'm sure there's more but I don't know
You can tell if a CRT is using a trinitron tube if there are two very faint and thin darker horizontal lines 1/3 the way up and down from the edge of the screen. When I first saw this, I thought my monitor was broken, but upon some research I found out that this is intentional...I think it has something to do with aligning the image due to having a perfectly flat screen over a curved one or something like that, I'm not sure (you can read about it more here http://en.wikipedia.org/wiki/Aperture_grille), but if you notice these thin lines it means you have a trinitron (apeture grille) tube (this may have been discussed on Vogons a while ago).
Other specs to look for is refresh rate at certain resolutions...I for one am fine with 60Hz when I'm using 2048x1536 (some people think it's too low, but I like having the high resolution to have more "real estate" on my screen when I'm managing my photos, for example) but the higher the refresh rate, the better in most cases. 75Hz is a good refresh rate to look for when talking lower resolutions than 2048x1536 or 1600x1200.
Myself and others have posted pics of some CRT's in this topic:Post pics of your CRT monitors if you want to get an idea of what's out there.