Is this for gaming? What games do you play and what kind of monitor do you have? Your computer is still pretty good.
I wouldn't upgrade your CPU. It's still perfectly suitable for modern games.
You can double your memory to 16gb if you want, but it won't make a huge difference just yet. Soon it will be more necessary though.
The GPU makes the most sense to upgrade right now, but depending on your games and monitor even that might not be worthwhile.
Got an SSD? That would definitely be a nice addition.