It would depend on a few things:
How big you want it?
How much money you are willing to spend?
What are you using it for in addition to video games?
You can get used 21" CRT computer monitors for dirt cheap in some places ($50 - $150) These can be used for video games as well as regular computer usage (surfing, text, etc)
If you want bigger you can try a presentation monitor (aka. multimedia/data monitors). These can go up to 27", 29", even 32" or 37"

. Used ones can be dirt cheep also, but may have limited resolution, and may or may not have a normal 15-pin VGA port. They probably have S-Video and/or BNC and component connections. Examples are NEC XP-2990, Sony PVM-2950Q, etc.
You could try a Princeton Arcadia
http://www.firingsquad.com/hw/2682/Princeton_Arcadia_27-inch_CRT/or
http://www.audioreview.com/cat/televisions/tvs20-29/princeton/PRD_123991_2724crx.aspxIt looks very promising, but also expensive!!!
Or you could venture into LCD land if you want to spend some $$$
I have no experience with LCD monitors for gaming...but they will play games and do PC stuff...
While I was in the store the other day, the salesman showed me some of the LCD TVs which come with every connector imaginable including a VGA port. so you could use one of those as a PC monitor and an HDTV if you wanted. 27", 32", etc. these are the widescreen type.
EXPENSIVE!!! No thanks, I don't even subscribe to cable. I don't care to spend several grand to watch snow in high definition!!!
If you decide to go the traditional route, you could get a used 25" or a new 27" for under $200. You use S-Video or composite to connect your PCs video card to most TVs.
I got a JVC 27" TV (AV-27530) traditional TV because I am using it in a dedicated setup and it was less than $200 brand new. This particular model has component RGB inputs, which are better that composite or S-Video. (Only the JVC and another Toshiba model have the RGB component inputs that I am awhare of)
This setup looks way better than composite or S-Video!!! I am really happy with this setup.
You would need a capable video card (Nvidia 6600, 6800 or Radeon 9550, 9600, 9800 etc.) plus a DVI to Component adapter and a set of RGB component cables. All available at your better retail or online stores.
Most or all of the flat-screen CRT TVs will have component RGB inputs, but are more expensive than the curved screen TVs. Plus I have read several posts by BYOACers saying that curved-screen TVs look better than the flat-screen TVs for MAMEing anyway.
I hope that helps...let me know what you decide to get

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