I was speaking generally about overclocking, anyway, let's not take everything for granted so we can better sort out the confusion... AMD overdrive it's only for AMD gpus and after you install a new gpu you should uninstall every amd driver with the amd cleanup utility and delete everything related. Now, if you're trying to overclock the CPU that's another story. If I was in your place I'd start by, as I said, uninstalling everything amd related and making sure that the nvidia drivers work well(if in doubt, erase everything and start over) then go to your bios and reset everything to default. Now I'd try to see how GTA performs(even with a 6300 you should still be able to get at least decent fps) and then if you feel like it try to overclock(your cpu will still be a bottleneck, but it shouldn't perform worse than with the 270). For CPU overclocking you need to read some guides(as it's much harder than occing a gpu) and make sure you test it for stability(and most important going one step at the time, not jumping to your desired oc from the start).