You have an available PCIe x16 slot, so no problem there buying the latest/greatest gpu's but the power supply is lacking to say the least. 300watt psu is tiny, so plan to buy at least a 650watt watt psu, mainly look at the watts on the 12v rail/s then divide by 12 to get the amps on the 12v rail, with your GPU upgrade. What games do you play now and what games would you want to play in the future?
Look here for the games you might want to play, look for at least ~60fps for a playable game:
http://www.guru3d.com/category/vga_charts/
And shop newegg, read reviews etc:
www.newegg.com
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