Is there any 3rd party Riser card (I forget the real name, the 2 cards you put ram on in the pro) That supports DDR3 Ram?
Nope.
The current (and all previous) Mac Pros use "DDR-2 FB-DIMMs". They aren't even "normal" DDR-2" They are a special variant of DDR-2 that has an onboard chip (called "AMB", the "Advanced Memory Buffer") that turns the normally parallel memory protocol into a much faster (clock-speed-wise,) serial connection. This means that many fewer pins are necessary for data; which means you can have many more memory channels. (Back when FB-DIMMs were new, two channels was about all any system had; FB-DIMMs made four channels easily possible.)
THEORETICALLY, it would be possible to make a new AMB chip that allows faster DDR-3 chips to reside on the module, yet it would connect to the exact same FB-DIMM connector.
The problem is that the connection from the memory controller to the AMB on the current Mac Pro is only fast enough for DDR2-800. So even if you threw DDR3-2133 on those modules, the memory controller wouldn't know what to do with the extra speed. (And, to my understanding, nobody makes DDR-3 FB-DIMMs yet, anyway.)
So it's a chicken-and-egg scenario. No FB-DIMM systems run faster than 800 MHz (equivalent,) so nobody makes FB-DIMMs faster than 800 MHz.
Edit to summarize: Nobody makes a replacement riser card; but such a card wouldn't even be necessary, since by its design, FB-DIMMs only need a new on-DIMM AMB to support a new memory technology such as DDR-3. But even if they *DID*, it wouldn't matter to the Mac Pro because its memory controller wouldn't support the faster speed.
It's like trying to run 94 octane gas in a Honda Civic. The engine can use it, it just can't take advantage of the higher octane properly. (In fact, using higher octane gas than your car needs actually *REDUCES* power and fuel efficiency.)
