We have released the best responsive One Page Checkout yet!
http://www.bcsengineering.com/store/smooth-checkout-responsive-one-page-checkout-for-x-cart.html
We've had such great response from our Mobility module and it's checkout that we've created it as a separate module for X-cart! Available currently for 4.6.x and 4.7.x, have the checkout flow you've always wanted!
Carrie