Frank, I just noticed this thread.
Yes, you can auth and then capture funds from within X-Payments via Quantum Gateway.
How to check what X-Payments can or can't do with regards to payment gateways:
Find your payment gateway at
http://help.x-cart.com/index.php?title=X-Payments:User_manual#Appendix_A._Supported_payment _gateways
Check crosses (or pluses) in appropriate column to the right of your payment gateway name.
+ - X-Payments can do that
- - X-Payments can't do that