Neither are 'tops'. If you want higher details and higher FPS both, then you should consider upgrading both. Between the two, I'd suspect your CPU is the most in need of an upgrade. The a8 is not a very powerful CPU.
If you want a 'cheap' upgrade then a used intel i5-2500k system is far more capable and very low price. It won't be ideal, but will be fast enough to not be the bottle neck. For example, the above named named CPU combined with 8gb of ram and a 970gtx can play fallout for on all max settings on a 1080p screen. With a older (and far cheaper) ATI 7950 video card you would need to turn down the settings to somewhere between "mid-level" and "max". I know you mentioned GTA V, but I don't play that and so the above is just to illustrate that once you get a CPU that is just good enough, the video card plays a far bigger roie in game performance. So while a Skylake CPU might be preferred, it is a luxury not a requirement for decent gaming performance.
If you want to squeeze a little life out of it while you consider things, look in to tuning your windows system. There are plenty of sites and tools for this. Short version turn off everything you can live with out (make Windows 7 ugly), reduce background tasks, reduce items that load on startup, and so on. This alone should give you a slightly more even gaming experience.