im gonna replace the o rings today on 1 of my fuel injectors cause its leaking and was wondering if it would be beneficial to go ahead and put new ones on all 4 injectors while i have it apart.also is there a gasket that goes in the middle where the intake splits on a 97 cavalier 2.2

Yes there is a gasket where you asked.
If you replace the O rings on that injector it will probably still leak usually those particular injectors crack and cause leaks insted of O rings so your best bet is to look really carefully at the injector itself and leave the others alone.