The difference between the web site that you showed us and yours is that, they paid to have their web site designed and developed exclusively for them. As an E-commerce developer, I can say that the shopping cart they are using is Magento Ecommerce.
and Word Press for their Blog.
Magento is vastly Growing in Popularity and only a handful of developers know how to modify the templates. As for the Word-Press Integration, you can install the modules that allow you to integrate the Word-Press Registration to the whole site...
My Question to you is why do you want to use Google Checkout? Most people don`t use that because there`s been flaws in the Code and People have been hacked.
See that it pays to have your site professionally designed? if you decide to go with a new media[shopping cart] you`re going to have to delay your launch date...
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!