Help! How to validate other site data before config option change?

Hello everyone,

My requirement is that let's say I have one configuration option called "personal computer" which is text box.
User will fill 1, 2, 3, etc...
When they check out ("Click to Continue" button), I need to check one value from my another site by calling web service.
Eg. If they fill 3 and result from another site is less than 3, they cannot check out.

My problem is I don't know whether there is a hook that run before user can checkout.
I had tried following hooks but still can't find the solution.
1) ShoppingCartValidateProductUpdate,
2) ShoppingCartValidateCheckout
3) ShoppingCartValidateProductUpdate
4) ShoppingCartValidateCheckout
5) PreShoppingCartCheckout
6) PreCalculateCartTotals

Most of them are not working, or, I don't know when will they fire.

Please help me. Is this possible to achieve?
Or is there any work around to achieve this?

Thanks in advanced
tslin