A 16x and a 4x wont do you any good performance wise.
The other guy had the right idea, sell your 3870s and get a 4850.
You arent stuffed with cash, but you are prepared to buy a open box item for $100.
Open box items are sent back because the last guy couldnt get it to work. There is a 15 day return on them and 0 exchange.
Oh not to mention you wont get the i/o plate or CD or any cables.