If you have two pcie x16 slots on your board so it can run crossfire, or sli, (make sure it can run sli and says it can since that needs two x8 speed slots or higher to run while crossfire can run on the slower x4 slots). A pair of used Hd-7850's or r265's is fairly common ( they are the same card so you might find a R265 and Hd-7850 together). Often people will sell a pair of cards for cheap. Some of them can be a good deal. Two smaller cards can run cooler than one larger card and can fit in smaller cases. These smaller cards were popular mining cards so there are some selling pairs of them for cheap. A pair of R260s, hd-7790, 7770 are ok too. STAY AWAY FROM ANY OLDER CARDS LIKE THE GEFORCE 580, 480, HD-6970, 5970 THESE ARE TOO OLD AND HAVE ISSUES LIKE FAILING FANS AND ALSO USE LOTS OF POWER AND ARE HARD TO KEEP COOL. THEY ARE FAST CARDS BUT THE WATTAGE THEY USE TO GET THERE IS NOT WORTH IT. A 750TI IS AS FAST AS A MASSIVE 480 OR 6970 BUT USES 1/4 THE POWER.
I don't have gta5 but do have GTA IV. With a pair of Hd-7850's and a A6-3650 4 core apu at 3.5ghz the game runs well. I still cant totally max out the settings so a few things are turned from max to high, or 16x aa down to 8x aa. I would suspect that the newer one, GTA 5 will need med-high settings to run on my system where GTAIV runs on high-max settings. I run at 1080p.
Do some research so you know what you are buying.