You didn't ask, but my opinion is that Google Checkout is not worth the trouble to add. We have GC, PP ECO, and a traditional merchant. Our merchant sales are about 60%, PP is about 40%. GC is like 1% or less. The "only" positive about GC is having that little cart next to your name if you advertise with Adwords or list in Google Base. And of course there's no fee just having it.