Zotac GTX 1050Ti Mini. It's the only gaming card that fits without modding. If you get anything else-I told you so. The 305W PSU is OK.
I'm assuming you have the tower, and not the silly SFF version. The DT you might work with some.Let me know if that's what you have.
You need to ditch the Q6600. QX6800SLACP (G0 stepping). It can be overclocked to about 3.45GHz using Throttlestop 6.00software, and heatsinking the 6 VRM MOSFETs. Follow the link in my sig. to an article about modding these.
Cooler D9729 is needed also. This will run 1080P at low-mid settings.
I'm not a gamer so IDK about GTA5, more likely GTA4 or older will play well.