Definitely the GPU. The whole AM3+ socket of that MB+CPU are outdated. It has trouble keeping up with Intel Haswell/Skylake i5, it will be superseded by AMD Zen on socket AM4 later this year/early next year. => Upgrade the GPU now is still worth the money (depend on what do you want to do), upgrade the CPU/MB is a waste of money (no matter what you want to do).
Graphic card that would be an upgrade are RX470-RX480 on the red team and GTX1060 on green team (GTX1070 would be a big waste on your current system, but you may be able to make use of it once you upgrade the system when/after Zen and Kaby Lake begin duke it out in a few months).