You're going to need to add some more specifics. What resolution do you play on/intend to play on? What frame rates are you shooting for?
If you're going to upgrade, you'll likely need to upgrade 3-4 parts: GPU obviously, but CPU, power supply, and possibly motherboard depending on your CPU choice.
If you throw a higher end gpu in, the CPU will likely bottleneck and your PSU will not be adequate enough to power the rig.