A CPOL is definitely a good first choice. Remember that to get the maximum effect from it (The deep blues and contrasting skies for which effect it's most often used) the sun needs to be low in the sky, and as close to 90 deg to your lens axis as possible The higher in the sky and closer to the sun, the less effect it will have. They're also useful for removing reflections on water and other shiny surfaces (The sky in this image is all as a result of using a CPOL at the opitmal time: http://www.rthtg.net/john/crete/Buil...%20(Large).jpg )

UV filters are relatively pointless on DSLRs except as an added layer of protection for the lens since there is one built in, and then, as mentioned, often cause vignetting at larger apertures.

If you're going to get another filter, I would suggest a graduated neutral density (G-ND) which will allow you to reduce the dynamic range between sky and ground in landscapes and scenes like that, resulting in fewer blown clouds and more foreground detail

Remember, filters are part of your optical process. In fact, they're the first thing the light hits, so DON'T BUY CHEAP! . Cheap filters will cause flare and aberration as well as hazing in your images. Tiffen and Hoya multi-coated filters are decent. Singh-Ray, B+W and Heliopan are even better.