you'd notice the second 7970 in xfire more then a new cpu. That i5 is pretty much the pinnacle of gaming CPUs... when overclocked.
In short, you probably will notice the improvement a 2nd 7970 brings, you won't notice a difference with an i7-3770k or i5-3570k. Your i5 is overclocked at 4.7, its doubtful you'll be able to overclock an i5-3570k or i7-3770k high enough to give you any improvement in performance... overclocked to 4.5-5.0ghz your i5 is about the fastest gaming cpu you can get short of $1000