contrast ratio on samsung lcd's

I'm new to the whole HD tv thing and am looking to buy a samsung 40 inch lcd. Amazon seems to have the best deals on these. All the ones I like have full 1080p, but they differ in contrast ratio. I don't even know what that means. They have a 15,000:1, 20,000:1, 25,000:1, and a 500,000:1. The larger the number the more expensive. How important is this spec and what would be the minimum ratio I should look for when making my decision? Thanks to anyone who responds.