What if Isengard joined forces with Gondor?

[The Might]

I am not really sure what you mean.
If Saruman wouldn't have rebelled against the rest of the Council, he would have never created any army, so Isenagard wouldn't really have had a big force.
But I am quite sure that he could have helped the Fellowship a lot.
Many things would have been different.

Gandalf would have had no need to go to Isenagrd, since Saruman wouldn't have asked Radagast to send this message in the first place. This way the problems caused by the Nazgul in Eriador could have been avoided and Frodo could have reached Rivendell much more easily.
There would have been no need to use the pass of Caradhras or the Mines of Moria, the Gap of Rohan would have been the best choice. Rohan's forces wouldn't suffer any losses and Theoden could reached Minas Tirith sooner then in the book. The Ents would have played no part in the war probably. Gollum would have died in Moria, and there wouldn't have been a Gandalf the White.

So yes, I guess if you think about it this way, you could say that if Saruman wouldn't have rebelled, the book would have had much less action. But in the end, I think it would have also meant failure for the quest. Frodo had reached a much greater level through all the difficulties he faced on his way, it was this that helped him overcome all the problems and make it to the Sammath Naur. Also, Gollum would have died in Moria, since there wouldn't have been any Fellowship to come through the west gate just as he was hiding around.

So, I guess that if Saruman would have remained a good guy the Fellowship would have failed in the end.