The wort chiller is great if you have a full boil going. If you are only doing a few gallons and topping off with water, you could boil that additional water ahead of time and refrigerate before you brew. Then add your sanitized water to the boiled wort to cool it (but not in a glass carboy!!)
If you are boiling the whole 5 gallons, then the chiller is highly recommended. And if you aren't boiling the full amount yet, then I suggest looking to do that soon. It will likely improve the beer.