For just enabling PhysX in a nVidia card , you need minimum of 32 stream processor and 9500 GT is just satisfying the criteria. If you use it as PhysX card then there is high chance of bottleneck. For PhysX, you need to have a card with minimum 64 stream processors. Get a 9600 GT or GT 240 1 GB GDDR3 version. GT 240 1 GB will cost you around 4.2K-4.6K.
Now if you ask for my suggestion then add another 9800 GT for SLI. two 9800 GT in SLI will offer you better scaling with PhysX enabled since the PhysX work will be devided to both the cards.
Will my PSU be able to support both 9800GTX+ and 9600GT? Also I opted for 9500GT because it fulfills criteria for PhysX and I was able to get used card (but still under warranty) for 1.8k. Used 9600GT is hard to find and new one is not worth investing for just PhysX capability.