Depends what you want to shoot I guess. My advice is buy a Cokin system. Get some NDs and Grads, and then when you get a second, third, fourth lens, you can get the adapter to make your filters fit them all.
I wouldnt buy a polariser from Cokin. Id get any brand (I recommend Hoya but any is fine) and get it in a 77mm thread size, and then off of ebay or amazon or somewhere cheap, buy a 52mm to 77mm step up ring so that it will fit your current lens. 77mm is the standard Nikon size for pro lenses. So buying this size filter you know you will never grow out of it like you would if you bought a 52mm polariser and then realise it wont fit your new 24-70