First thing I'd like to say is that you might not see much difference with or without the card. When you installed the 275 and ran the drivers it already has physics enabled and the card is fast enough to handle most physics without a big hit. i had a similar setup with a 9600 and it's not in my system anymore.
Secondly, If you want to install the 9600 or physics ready card all you need to do is install the card and boot up. Windows should find it. Then open the Nvidia control panel to see if the 9600 is there and is set to be dedicated physics card..... it should be but if not just put a check in the box next to it.