I've got this device called a Kramer FC-14. It's a RGB to Component transcoder. I can use RGB cables with my retro consoles on my plasma, and the picture quality is really nice. Only thing is, with a modern display, you're talking about a widescreen display, and with widescreen, you have a compromise to make, because either option you choose, you're going to deal with an unfortunate side effect.
For example: Pillarboxing
When my plasma is in 4:3 mode, it has grey / white bars on the sides of the 4:3 area. obviously, this is pretty distracting, and not exactly ideal. Also, if you did this for extended periods of time, you'd really have an image retention problem. with the sides constantly in that white / grey mode.
Another Option is to use a Zoom mode:
The zoom mode will fill your widescreen display, but it's going to distort the image in one way or another. When I play in this zoomed out mode, sometimes health bars and other information is zoomed off the screen, and I'm kinda walking blind. Still, I think it's a bit better than the normal 16:9 stretch mode that just makes everything short and fat. Of course, neither option is ideal. Wouldn't it be awesome if they made a 40 inch 4:3 plamsa ? It sure would, but unfortunately, none are currently being made. So... back to my widescreen:
I still really like the RGB picture going through the Kramer FC-14, but damn, those white / grey bars are super annoying...
I took some black fabric, and draped it over the sides, just to hide the white / grey pillarboxing. It's a bit of a quick and dirty method of blocking out those sides, but it's better than nothing. I can play like this on the living room plasma, if my room is dark, and enjoy it like it's a 40 inch 4:3 gaming monitor. Only thing is, I'm not sure that I want to be screwing around like this with the living room plasma. But it did give me an idea... Sometimes I see big plasmas on Craigslist going for cheap prices. It might seem weird, but somebody could buy a widescreen plasma, with the idea of using it exclusively as a 4:3 display. A 50 inch plasma is basically a 40 inch 4:3 TV. Most people using fancy Sony PVM Monitors are gaming on a 20 inch monitor. This is 40, so you're talking double the viewable real estate. I'd still prefer a high-end, late model Sony PVM, but using a big plasma as a dedicated retro gaming monitor could actually work. Especially if you could build a really nice contraption around the thing that completely hides the non-viewable area of the plasma.
Basically, you just don't worry about the fact that the sides are going to burn in the wrong way, completely uneven. You're dedicating the plasma to this specific use as 4:3 gaming monitor, and when it no longer has use in this regard, you're just going to have to consider it a total loss. Give the thing away to somebody on Craigslist, or just take it to an ewaste place. So, you don't want to spend a ton of money on this. You just want to find a good plasma, that you can get for dirt cheap.