It's possible to do that (use an Nvidia card for dedicated PhysX with an ATI card as a main) as long as you're using Windows XP or 7. With Vista, it's my understanding that it won't let you have both sets of drivers at once. Anyway, it shouldn't be an issue for your system, but read one of the many tutorials on how to set it up correctly before you just go installing things. You may need to perform a few tweaks.
By the way, I used to have that same exact motherboard, and it was a real piece of poo. Intel boards tend to freak out and not work if you use anything other than standard-issue components at stock settings. Even with standard parts and stock settings, you're lucky if your system is stable. So I guess what I'm trying to say is, if you try this and it suddenly doesn't work, don't be shocked.