Hi Dave!
What you want is already implemented in X-Cart 5.0.9.
So, what I would suggest is:
1) Check out our latest X-Cart 5.0.9 version on your test server. You can download it here:
http://www.x-cart.com/downloadxc5beta
2) If it is OK with you, you can upgrade your X-Cart Next installation from 1.2.3 to X-Cart 5.0.9 simply clicking the "Updates are available" link at the top of your admin area and it will add new checkout to your store.