Currently, there is a XML payment module for PSIGATE you can download off Zencart [url removed, login to view] that has last been updated to work with version 1.26. While it does process transactions for Zencart 1.51, it does have issues:
1) User is getting a blank page responses when a transaction is not approved (ie using an invalid credit card is an easy way to test that)
2) Transactions are being approved even if the CVV2 code does not match the actual credit card
So we need to get these items fixed as well as any other possible issues (that we are unaware of) so we can get this module working properly for Zencart 1.51.