As the supply is outside the EC, I would say that VAT is not chargeable as it is outside the scope - this is under the electronically supplied services.
If within the EC, then so long as the advertising is for a business, then it will outside the scope nad the customer company will charge using reverse charge. However if non business customer, then VAT will need to be charged.
My advice is that as your income is from the USA only and your business is UK based you would not be liable to pay VAT as you would not charge VAT for goods/services sold to the USA.
Yes, as you have reached the threshold, you will need to be registered. Just go for the standard, as you will be able to reclaim your costs in the uk, ie ISP, Tel etc