I have a number of high-quality ribbon microphones, and I love them, but I have to say ... get the MD 421. As others have said, your money will not buy you a really nice ribbon mic.
The 421 is the Swiss Army knife of microphones. There is hardly anything it won't do a pretty good job at, and it does a fine job with vocals and guitar speakers, not to mention drums and woodwinds, esp. saxophones.
I have a pair of 421 Mk II, and I hadn't used them for a while. I broke 'em out for a live gig last week and was reminded again how much I like them (a lot).
I have a videocassette of clips of live Beatles performances, and in general the sound sucks. But there's one clip where they're using 421s as vocal mics, and the sound is very good.
The RE20 and SM7 are very good mics but are not as versatile as the 421 IMO. The 441 is an excellent mic but out of your price range. I have them all.