I have had this motherboard for a while, and I recently upgraded to a gtx 680. I know this board is not capable of SLI, but since it has an additonal pcie slot, would it allow the use of a gtx 550 (on sale for $89 on tigerdirect) as a dedicated Physx card? or would such a card even be compatible for that type of use with the 680?
You don't need a PhysX card to supplement a 680 even if it did work.
I don't see any reason it won't theoretically work, although there may be a practical reason that I don't know since I am not a huge expert on this.
In any event, PhysX is pretty much dead by this point anyway and its probably not going to bounce back in the future either.
In times past, when PhysX was used more and when video cards were a lot slower it made some sense to offload some stuff to a 2nd card, but these days the overhead is probably more than the benefit even if you get it working optimally.
See here for a list of PhysX titles old and upcoming