weigh your cranks, then compare the weight with some of the cranks here http://weightweenies.starbike.com/listings.php

Then subtract the difference from the overall weight of your bike, and ask yourself wether it will really be noticeable, and or worth the money.

You'd probably need to swap out most of the components on your bike to make a really significant difference in weight. The single most noticeable thing you could do would be to replace the wheels. You could also lose about half a pound trading your brooks for something like a Flite. After that you're just penny nickel diming it with a few grams here and there that will eventually add up to a pound or two depending upon how much you spend.

P.S. your seat keeper chain probably weighs more than the difference between those cranks, and expensive light ones. You want a free weight reduction, pitch that thing and replace it with a thin cable from the hardware store. It will be just as secure, but will weigh 2 oz instead of 6-8