You'll need that quad core and 8gb for a game like GTA. The three best GTX cards that don't need a 6 pin connector and will work with a 300W power supply or more are, from slowest to fastest: GTX 750 Ti, GTX 1050, GTX 1050 Ti(some models). The GT 1030 is a possibility, but only at 720p.
If you could find a good deal on a 750 Ti I think that's the best match for your system. Older GTX cards might cost less and give you more performance but they will almost certainly require at least one 6 pin connector.