If the camera is still new to you (as you are to the camera), why not go out and shoot some pictures to see if your setup can take the landscapes that you desire? 18mm on your 500D should be ok for normal landscapes, and the longer 55mm end should provide some isolation of individual elements of the landscape.
Later if you feel the need for even more breadth, you can explore the range of ultra-wide lenses that go into 10-12mm focal lengths, but those are not for everyone. No need to jump on that boat before you finish exploring what you have.
Rashkae, well said.
Doesn't that sound just like me when I started?
ngwixn, yeah, go ahead and shoot some landscape and I am sure you will find them improving week to week with your kit lens at its 18mm.
Once you kena poisoned with Gear Acquisition Syndrome, a lot of time of time is spent on browsing lens brochures, personal classified ads - Time that you can better spend honing your skills. For landscape photography, browse a lot of landscape photos - what works, what doesn't. Borrow books from the library, sign up an account in www.scribd.com and search for landscape photography (don't want to over-elaborate here but you can get a lot of free ebooks and advice - Hint hint).
There are a wealth of resources if you know where to look.
But yeah, agree with all the above - shoot more. Gear comes later, Art first.
I guess NM is referring to without mounting a lens, you can see the mirror flipping up and down thru the lens mount.
anyway, my point is why some people does not regard kit lens is a very useful lens, it cover the most widely use focal length of the general subjects, a pro f2.8 lens also cover the same focal length too.
if a photographer can't shoot well with a kit lens, getting a better lens won't help much with his works.