nVidia GeForce 6600 compatible with Mac Pro?

I have a Mac Pro (2006 - 2007) that already has an nVidia 7300 GT graphics card installed. With Snow Leopard released, and my Intel Xeon CPUs becoming let's say, tired, I want to be able to maximise the speed of my Mac Pro. I have Snow Leopard installed.

I want to be able to install a secondary graphics card to run aside the 7300 GT. I'm trying to install the nVidia GeForce 6600 with 512MB of VRAM, but when I turn on my Mac it keeps restarting with the chime going off every ten seconds. I can't see anything on my monitor while this is happening.

I've tried taking out the 7300 GT and just having the 6600 GeForce card installed. The Mac starts up and goes into my user account as normal, but there's nothing on the monitor.

The graphics card was previously installed on a Windows machine.

Is there any way that I can get it to work on my Mac Pro?

Thanks