Has anyone had the opportunity to run their ecommerce store a while then latter on install Google Checkout or Amazon Payments? If so, did it increase conversions and/or lower cart abandonment? I ask because I am about to launch my store and currently I plan on only using PayPal for a few months but I know how incredibly limiting using only one payment type is. However, adding either GC or AP really mucks up my design flow. blah.
Keep in mind when you sign up to use GC or Amazon Payments services these entities will hold any payouts due to you for a prescribe period since you'll be considered a new business utilizing their services.
They may help depending on what you sell. We typically see something like 80% of payments made directly on a credit card, 15% through paypal and 5% through google checkout. I recommend accepting credit cards directly and accepting paypal. There are a ton of people that wont use paypal at all and there's a ton that prefer it. Offering both methods is the best way to get the most out of your shoppers. Beyond these 2, I think the increase in conversions is negligible for most sites.