A friend of mine has a Sapphire R7 250X and is pretty happy with it. But he prefers to play somewhat older games and is not so much into shooter and racing games. (Where afaik high frame rates matter the most of all genres)
But yes, depending on your preferred settings and the games you are actually playing you might look up to something like a GTX 750 Ti - 950 or a similar performing AMD card.
And you should totally get more RAM. preferred 8 GB or the closest you can get. If you are already on DDR3 that might be easy but with DDR2 you might take a look at the prices first, before considering buying.