Many arcade monitors can do 1080i (some can do 720p, as well). Some newer ones are available with flat screens. They'll have RGB/VGA inputs, but nothing else. Could be quite an attractive solution, depending on what you want to use it for. Arcade monitors tend to be reasonably competitively priced with similar HDTV offerings, but are always 4:3, while most HDTVs are going to be 16:9.
Of course, the dreamcast isn't HD (it's just progressive scan 525 line, aka "EDTV").