I play a lot of ps2 on fx6300 @ 4.6GHZ with r9 290 & if I ever drop below 60fps it is ALWAYS CPU bottleneck... the majority of the time I'm locked at 60fps on a combination of mid and high settings though. I suspect you will see some bottlenecks in places, but it wont be too bad - itll depend on game and scene.
EDIT: Saying that, if you;re upgrading GPU, frames will never go down in games, they'll either stay the same or increase. for the upgrade.