When I changed from a 50hz Sony CRT to a 100hz Pioneer Plasma this year I got a definite improvement in standard definition pictures, both digital and - surprisingly to me at least - analogue. It is switchable to 50hz and when I did this out of curiosity there was a definite decrease in picture quality on the analogue sources, which I still use sometimes.
So I would definitely say to go for 100hz on whatever set you buy.
BTW, don't know what size set you have right now, but I found that a 37" plasma takes up very little extra room over my old 32" CRT. The screen size is pleasantly bigger but the width and height actually didn't vary that much. It doesn't feel too big for my room, and I'm only a few feet away from the set.