It should work fine, Boozybob, but it's best to ask the vendor if their cards can be used online. Several US users have reported success using this type of Visa cards. I have used a Visa debit card to purchase WGT equipment several times here in Europe, too.
(A card you'd like to use online should at the very least have the 3 "security digits" on the back. If it doesn't, it can't be used on WGT.com.)
And, you can also use PayPal. One of my cards (a MasterCard) got rejected by WGT, but it's accepted by PayPal, so I can pay with that card, too, it's just that PayPal is put inbetween the card and WGT.com to act as "intermediary".