Need Help Upgrading My Mac Pro

[Stixx31]

I bought used from a friend of mine his old Mac Pro 3,1. It came with an ATI 5870, and dual 2600XTs. Now. He was a general user, I'm a video editor and gamer, and I need an Nvidia Card for the CUDA Cores. I had found this
http://www.ebay.com/itm/150985668474...84.m1423.l2649
And thought it was the best deal but Now I've read you can put PC cards in a Mac Pro, which would be a much better deal for me. So My Questions are this-

1. Installing the best graphics Card to use on Mac and Windows for Mainly Editing but also Gaming
2. Adding Wi-Fi? I've read that you can use an Airport express card but every guide I've read contradicts one another
3. Enabling Cards to use CUDA Cores and Effortlessly switch between windows and Mac. The Idea of the Black Screen doesn't appeal to me.
4. Anything Else I may need to know.

Any and All Help is Appreciated