We're a UK-based web design agency looking to implement a bespoke shopping cart solution for several client sites. The shopping cart we can do, I just want a simple payment-processing utility that'll accept major credit and debit cards.

The only requirement is that it must be a seamless, AJAX-style process. I don't want customers filling in the payment form and then landing on some 3rd-party page, no matter how well branded it is. It must all happen within the client site.

So: I'm looking at Worldpay, and Protx, and Netbanx. These seem to be the major players and seem to offer what I need.

If you have experience of any of these and can give an indication of:
1. How easy integration is
2. What their support is like
3. Overall value for money
I'd be proper grateful, like

And if you know of any other major providers that I should look at, I'd like to hear about those, too.

Tanks