The P5B is a 965 chipset board. While it has two PCIe slots, they run at 1.0 PCIe speeds. Not only that but it's setup for 16x/4x. This means those two cards will be running at 4x/4x speeds, and nearly all of the time will run slower then a single card at 16x. I wouldn't get a second card and run it on that card.
Edit: Wouldn't run it on that board I mean of course...
The blue one will always run at 16x mode. The problem is once you add a card in that second slot, it will only run at 4x mode. It doesn't matter now that you've linked them if the blue slot runs at 16x or 1,000x. Because it's linked to a 4x slot they must work together at 4x speeds. 4x/4x PCIe 1.0 is slower then 16x PCIe 1.0. As such I wouldn't buy another 4870 to run CF.
If you want more speed look at buying a 5850 or 6870, etc. These cards are faster then your 4870.
Edit: I wouldn't bother with CFing your 4870. Older slower tech now, not worth the time.