The 6400 is a low power processor thats really meant for work computers that need a quad core. The 6500 is a bit faster in games due to its faster cores and turbo speed, probably why you're bottlenecking in GTA V (but GTA V can bottleneck a lot). Depending on the net cost (ie what the upgrade cost is to you - the sale of your i5 6400) its worth it to swap out. Basically if it will net cost you like under 50 bucks, sure, but I wouldn't go dropping more than that on it.
What GPU are you running with this?