We had a discussion about pcie a few days ago. its impossible to swamp a pci e slot with either of those cards. You can put two 4870s in CF on a P45 and not have bandwidth issues. it's that fast.
And whether you think the 9800GTX+'s are faster means nothing. There are benchmarks to prove otherwise. The 4850 is higher on our graphics ranking list for a reason. Because there is evidence that says its better.
And the TP45 is 16x 8x anyways. Dont argue that point, I just checked the manual.