Bottleneck? No, not really.
Come on, a good dual core running at 3.1 ghz is NOT going to bottleneck a couple of 4870's in Crossfire.
Now, will the 4870s scale upwards and provide more performance if the processor was faster?
Yes, they will scale nicely if you do overclock, but that processor is fine for putting a couple of 4870's into crossfire.