Cards that max out bus speeds?

Wasn't sure whether to post this in the MB section or the video section but here goes...

I recently purchased a new AMD MB and a Radeon 3870. Unfortunately, I bought the video card 5 days before the GeForce 9xxx series came out and paid a hundred bucks more than they were worth a week later. However, now that the price has dropped so radically I was looking at getting a second 3870 to crossfire. While researching this I discovered that my motherboard will automatically drop the speed of the first PCI express bus from 16x to 8x.

My question is, do the current generation of cards, (the 3870 specifically) max the PCI express bus speed? If I buy a second 3870 to crossfire will I basically be throwing my money away by halving the speed of the primary card?

Any help would be appreciated. Thanks in advance.