One thing many online shoppers enjoy about paypal is that you can pay without ever giving your credit card number to the site, which is something that I believe helps many starting sites. However, I agree with Joe, the more options you have for payment the better off you are. Right now your using Google Checkout and Paypal, which are both great and Google Checkout definitely has the ability to give Paypal a run for its money. As for the credit cards, it looks like your using Authorize.net, which I`ve used for awhile now and think they`re a good company and their support team has always been very friendly and helpful when I`ve needed them.
I think you`ve got a pretty good combination as it is.
My only suggestion would be to add your header to the prestostore pages, as keeping a pretty unified look from page to page can be aesthetically pleasing to visitors and helps them know they`re still on the same site.