I am trying to set up a new storefront with both Google checkout and Paypal, and the only conflict I have seen so far is with Paypal Express checkout and Google Checkout. Both modules seem to want to use the same areas of the checkout page, and I have not been able to get them both working at the same time. I can get either one to work without much issue, but not both at the same time.
I am sure someone who is more familiar with PHP could fix it. I am an experienced developer with C#, but I am very new to PHP.
It appears to me that either the developers who worked on the Google checkout module did not understand how osCommerce works very well, or they intentionally set it up so that if you use it, it would be difficult to continue to use Paypal, too.