If you don't mind spending the money, save for a good zoom or prime
lense as it's worth it for sharpness. But you'll probably want something
wider than the 20mm, which will only give you a 32mm wideangle view due
to the 1.6x crop factor.

Personally, I like the 24-28mm range for landscapes. You may want to
consider the 17-40mm f/4 L lense...it's about $200 more though and would
give you a 27-64mm field of view. It would easily be the best of your
current lenses too.

I suppose it depends on how much quality you need/want for wide angle
shots as to whether it's worth it to you to spend that much. For me,
wideangle and telephoto is more important than the midrange. I happen to
use the same 28-105 f/3.5-4.5 as a walk about lense for candid snaps at
many events. But I pull out the L glass for important wide and tele
shots.

I don't have any personal experience with the 20mm, so I can't comment
on it directly.

But for the price, the 18-55 isn't half bad. It's not as sharp as a good
zoom or prime lense obviously, but if you're not doing a lot of close
landscape shots, or lots of groups where fine facial detail is needed,
it should do the trick. Just stop down to f8-11 to improve the sharpness
a fair bit and have at it.

Based purely on your current lenses, I'd say you'd probably be happy
with the 15-55. But if you're unsure, grab a flash card and take it in
to a store to test out the lense - the beauty of digital is the
convenience. Take the images home and examine them, then decide if the
18-55 is good enough for you.