A PCI-e 1 16x slot is sufficient for a 4870x2 or any other card out there at the present. It isn't until you start running multiple cards where your PCI'e gets cut down to 8x that there maybe potential issues. On a PCI-e version 2 board however a 8x slot has just as much bandwidth as a 16x PCI-e V1 slot.
In reality games really don't push the interface that hard that it becomes a bottle neck even on a 4870x2. The only time you may experience a bottleneck is potentially on some GPGPU applications.