is this easy to do and would it make a big difference to performance?
		
		
	 
Very easy to do, you just use the Nvidia Control panel to use your 260 for physx.
It would be a huge improvement on your physx performance to what you have now. It may also increase 560 fps on physx titles, as it wouldn't be doing any physx at all.
The downsides are: 
I imagine you may need a new psu to power both cards. You may have to factor in the cost of buying both the 560 and a new psu. 
Is your case big enough with decent airflow? It's going to get toasty in there.
Do you want a higher power draw? It's going to cost more to run your PC if it's on 24/7? 
If you care not a jot about power draw, then strike the last one off the list!
If It was myself in your situation I would probably lean towards doing it, though I would maybe try and flog the 260 and get one of the cheap 96/9800's to use instead.
Some will probably say sell it because there are hardly any games that use physx.
I'm just gutted because if the PhysX mod still works when the Batman: A C comes out, I'm going to have to rip out one of my 6950>70's out of xfire to put my 9800(which I kept to use as for dedicated physx with a 5870)in!
Batman: A C is getting spin that there is more use of physx this time round and if true then I'm having some of that.
That's why we love PC gaming.
 
So that we can use our old gpu's if we want to because we have a passion for PC gaming! That's why we won't be playing it on consoles. 
We want to see certain games in all it's marvellous glory!