A Physx card does not need to be a very powerful card. However, as you are going to watercool it and need to find a waterblock for it, it is hard to find a waterblock for a mid range card. So, I think the approach is to use a relatively more powerful card (for the sake of easier finding of a waterblock) of an older generation (for the sake of possibly cheaper one). This should then keep total cost (card + block) minimized.
For example, I think GTX285 or GTX460 are both good Physx cards to go along with your GTX560 Ti 448 SLI for Physx and their waterblock should be easier to find.
As an alternative to further cost down the cost, there is yet an alternative. Assuming you are using the Asus Rampage IV Formula (with four PCIEx6 slots available) and assuming your two GTX560 Ti 448 currently sit in the 1st and 2nd slots, you can move the 2nd GTX560Ti 448 to the 3rd
The motherboard can x16/x8/x16. So, the top card (at slot 1) and the bottom card (at slot 3) will still run at x16.
In this configuration, you will need to connect
the middle inlet and middle outlet of the EK 3-way SLI waterblock, eg by perhaps a pair of fittings. See pic below for the idea.
The water flow should work for both Serial and Parallel versions of the EK block.Edited by windfire - 5/4/12 at 11:08pm