The only Sage Pay option that is available for eCart 5 is the vsp form method used with Protx. This is a remote checkout that required the user to visit the gateways site to make the transaction. After the transaction is complete you can get back some details for the order.
You would only be able to apply this with the checkout wizard, adding this as a checkout will cause a bit of the PowerStore functionality to stop working in regards to the shipping that is setup by default and the way the orders are stored in the db. You can store the order info in the db by applying the store order summary and store order details manually on your checkout success page. The other part of the checkout that would not be the same is forcing the user to login.
