For the gams you are playing, I would suggest a CPU upgrade, depending on your budget an I5 would be great for you, I would suggest a 4th gen to keep the pricing low and would increase the performance in the games you play, CSGO is quite a CPU intensive game and I think it is important that you get the most for your money, If you played games like crysis, you would benefit more from a GPU upgrade.
Here are some suggestions;
I3 6100, £100, Motherboard ~£50, New ram ~£50.
I5 4460, £150, Motherboard ~£50, I would suggest this personally.
Going used.
FX 6300, Can get CPU and Motherboard for ~£150.
If you are fixed on getting a GPU however there are a few I would recommend,
GTX 1060 3GB £200
RX 470 £150 <---- Great value at the moment.
GTX 1050 ti £150
I would recommend a new CPU personally but the RX 470 is a great bargain, go with what you personally feel will suit you the best.