Not to be a downer...but with a 570 as physx with two GTX 690's would slow those two 690's down. you would get better fps and performance with those two cards handling the physx.
Besides, The number of games that actually uses the Physx Engine are very few and far apart, and even more so, dont need all that much power to handle the physx.
you WILL have refresh rate, or somewhere around 600 fps on current gen games with vsync off with those two cards at max settings(dependant on the game of course, metro and witcher would be like 120 fps at max.)
Just consuming more power from the PSU, wasting money in electricity.
Even if you WERE ready to dump $1200 on cards, I take money saving seriously, and try to show people the waste in the money.
That being said, a 570 for now is 100% fine, I just think using it as physx later on is a waste of resources.