adCenter community is a great site run by MS; worth the visit and here's some good articles to get you started.

I'm still not positive about this; but here's the numbers about Google Checkout from Google.

Google has added claims to its Google Checkout Merchant homepage that having Google Checkout as part of your AdWords ads will lead to higher conversions and a better click through rate. Google specifically writes that conversions will increase by 40% and the click-through-rate on your ads will increase by 10%.