If you can find one, I'd recommend the Mitsubishi Megaview CRTs. I have a "Pro 27" XC2935C and it works great. It accepts all kinds of resolutions, you can even do 720p albeit letterboxed. 480p looks awesome on it, and it has VGA connectors so you can plug the Dreamcast right into it without issue. I don't think you can do Component video, it's never worked for me at least and doesn't seem to be supported in the manual. But you can do analog RGB just fine.
And if you want to do emulators for something like arcade games, if you have a video card supported by Soft15kHz or an ArcadeVGA then you can use custom resolutions and get games running in their native video modes.
I also have a Sony PVM 2030, it only does up to 480i but it's still a great monitor, probably better than the Megaview for SD content. S-Video looks awesome on it, I never understood the jump between Composite and S-Video until I had one of these. It also supports RGB through a DB25 connector, but there are PVM models that take separate BNC connectors for the red, green, blue, and syncs. I also play a lot of NES on it in Composite (may do an RGB mod in the future), and while it certainly can't compare to RGB or a nice S-Video signal it's still a very nice picture all the same.
If you're interested in reading more about that kind of stuff, this forum is a great place for information and to get your questions answered:
http://nfggames.com/forum2/index.php?board=5.0
But long story short, AppleQueso and ATARI800XLfan nailed it. Just hit up your pawn shops/thrift stores/garage sales and look for a TV with Trinitron branding and at least S-Video (Component is an excellent bonus), I think you're guaranteed an awesome picture at a likely reasonable price if not free.
Here's some crappy shots of M.U.S.H.A. emulated on the Megaview vs. a model 1 Genesis through RGB on the PVM 2030, you decide which is which!
