An 9800GTX+ as a Physx card certainly will help a GTX470 main card.
8800GT has 112 shaders while 9800GTX+ has more (128 shaders).
Here is a most recent study by brettjv who paired up his 8800GT (Physx) with a single GTX470 and then with a pair of GTX470.
For both cases, there is 'significant' framerate improvement. Go and read the results in the thread to see if you are happy with the improvement.
I have looked at your motherboard (DFI Bloodrage GTI X58) which has one empty slot between the GTX470 and the 9800GTX+ and so ventilation should not be a problem.
Your PSU (ToughPower X7 775W) also has enough native PCIe connectors to do the job.
To install the Physx card:-
1. Power down your system
2. Insert the card into your motherboard
3. Connect the two PCIe power cables to the card
5. If the Physx card is detected properly, then go to NVCP to enable it as Physx card
6. If the 9800GTx+ is not detected properly, I would re-install the Nvidia driver again. The driver package should already be somewhere in your pc already. A Nvidia driver package includes the Physx driver inside already. Use the 'Clean Install' feature