So,as title says i can't choose which GPU to buy.I have 500w PSU 80+ bronze,4gigs of ddr2 RAM on 1066 and ddr2 MB also so as intel core 2 quad q9300.I have money to buy RX 570 4gb but i think it is an overkill for this PC,to mention that i will play games like PUBG,CS:GO,LOL...Is it better to buy rx 570 4gb or rx550 2gb as i said not for some hardcore gaming 
Thanks for reply.

Thanks for reply.