I recommend that if you are getting into filters for artistic purposes (i.e. anything beyond using a clear or daylight filter to protect your lenses), that you look into getting a Cokin P system, comprised of a holder and rectangular filters.
For landscapes you may want to have a look at ND (neutral density) filters; homogeneous to allow use of very long shutter speeds or graduated to balance between light skies and darker grounds. I use the Lee filter system, which is not cheap but very good. You could have a look here: http://www.leefilters.com/index.php/camera/ndgrads to see what it can do for your landscapes.
I actually have some Lee filters that are compatible with my Cokin holder, in my mind the two systems are interchangeable for the most part, and Singh Ray makes good ones too.
First off, a system like Cokin or Lee is pretty much the only way you can use graduated ND filters properly. I recommend getting both soft and hard edge, and maybe reverse grad ND if you like to do sunrise/sunset landscape shots. Two stops is a decent compromise.
Another factor in favor of Cokin for me is that I have lenses with 77mm, 67mm, 62mm, 58mm, and even 52mm with the new EOS M. With a Cokin P system, you can buy the filters once and get adapter rings for your lenses. They are a bit bigger than the screw in ones, but holy cow, you could go bankrupt buying good quality circular polarizers for your whole lens collection. With Cokin P, if you get a new lens with an odd filter ring size, your total cost to adapt your filters to it is a $20 ring.
One thing to consider is that the P series doesn't work with the widest angle lenses. I don't have a 16-35mm or 17-40mm, and I think they work down to about the widest on those lenses, but I am not sure about that.