You can still use the 9800GTX+ for PhysX. That will only give you any performance boost in games that use PhysX specifically. Most games, you won't see any difference.
I have a GTX 470 w/GTX260 as PhysX. The GTX260 sits at idle 90% of the time.
So it's probably just a waste of power.
FYI: new 9800GTX+ are selling on eBay for about $100. And keep in mind, eBay and Paypal both get a percentage of your sale if you sell through eBay.