I've seen benchmarks showing stuttering issues on GTAV with 4gb, so I think for that game at least you'll want 8gb of system ram. For a graphics card, a GTX 950 is probably your best bet. Considering your CPU, there's not much point in spending more money on a more powerful card. This assumes you have an adequate power supply.
In terms of general performance, not gaming specific, an SSD does wonders to make an older system more responsive. If you don't have one yet, that should be on your list.