I think the better question would be why is a 4870x2 going in an x8 slot?
If it's for CF quad, then the point is basically moot - you should be prepared to hit bottlenecks left and right in a quad GFX setup...
If it's for an old mobo (were there ever any mobos that had an x8 PCIe 2.0 slot only?) it's likely PCIe 1.0 x8, which means it'll be a fairly significant bottleneck. However, since it's such an old system, I would imagine your CPU would be the bottleneck there, not the bus between them.
If it's for a dual monitor/dual gfx card setup, then the point is moot again. There is no way that the bus is going to be the bottleneck even if you have 2x 30" monitors hooked up running a separate game (or openGL app) in each. The drivers are likely going to stop you from doing that at any decent speed, and even if they didnt, the CPU or ram would likely be the bottle neck.
So I have to ask - why is this question being asked :S