I agree with bladerunner6, save up until you can get a full front 3. If you watch Frys they usually get the Infinity p363 towers for $99 every month to month and 1/2 as a sale. The Center usually goes for $199 I think but not at the same time as the towers. If you can't do towers, I personally own the P163 and it's an awesome little budget speaker.
As for Polk, watch their ebay store and newegg.com.
Also what receiver do you have and what is your current budget (or better yet, if you save up for a bit, your expected budget)? Knowing that will help folks here provide better suggestions for your needs.