for the best performance you will use 9800GT. but to use 9800GT you have to make sure your PSU will be able to handle GTX560 and 9800GT at the same time. unlike 9800GT, 8600GTS doesn't need supplementary power connector to work. that's why some people like to use GT240 as their dedicated physx since GT240 is the fastest nvidia card without the supplementary power connector to work
My first question would be do you even play any of the few games that a dedicated PhysX card would work in? There aren't many. If you don't play any of them then don't bother having the dedicated card.