You can run two (any size) cards in full 16x/16x. Alternatively, you can run 3 cards in 16x/16x/4x. All 3 PCIex slots will fit dual slot cards, they are spaced accordingly on the P6T, not like the deluxe. That being said, you won't be getting much out of the 3rd card running at 4x, so stick to 2 cards.
Nice, let us know how the H2O cooling goes. I'd love to hear how high you can get that CPU on water! Have they started making waterblocks for the GT200 GPU's yet?