The way I see it, is that you currently have too many high gears.
I think that if you
could go to a 22T ring, you'll have the opposite problem with having so many non usable low gears, that you'll end up using the 42T ring anyway, to avoid frequent "inconvenient" double shifts.
Also, with a 20T difference between rings, shifts would probably be a little "ugly".
Wouldn't something like 30-42 make more sense?
Use this link to determine what you actually can use-
http://sheldonbrown.com/gears/