If you're already using nvidia cards for your main graphics, it's likely that adding a dedicated PhysX card will do exactly dick.
Here's a thread full of people on the Nvidia forums who tried that, and the conclusion is that in the majority of cases, you get no performance gain and the primary difference is that you use more power and generate more heat:http://forums.nvidia.com/index.php?showtopic=143047&st=0
Basically, if your main cards are anywhere near decent, that can handle PhysX on their own more efficiently than if you involve a second card. If the second card is inferior, you can even slow down the whole process.
Not to mention, if you are using three cards, most motherboards will downgrade the last two slots from x16 to x8 or x4. That probably won't matter with a pair of 250s, but you certainly aren't doing yourself any favors.
Basically, dedicated PhysX cards are for if you have an ATI card as your main graphics card.