The term "bottleneck" is so overused it's crazy, and there's really no way to answer it definitively since some games are more heavily GPU bound and others more heavily CPU bound, and if the game doesn't effectively use more than 4 cores there really won't be any difference since the 2400G and 2600 are basically the same processor with the 2600 just having two extra cores.
So, no, don't do that upgrade.