Before you decide on a gpu, you are going to need to upgrade your psu. You MIGHT get away with an HD7750 on the stock psu as it does not need additional power, but your 300w psu is not well made and you will run a risk of the psu blowing.
Anything over high low end video cards will be wasted on the CPU, you can get a Radeon 6670 that will run on your power supply and will run with the A8 video in crossfire for a decent performance boost over the stock A8. I would think anything over a Radeon 7770 will start to be bottlenecked by the CPU.
Your best budget upgrade is probably the 6670 running in crossfire. A 7750 card should run without any power supply upgrades, but you will see about the same performance as a 6670 in crossfire with the A8. To run a 7770 you will probably need a power supply upgrade, and I would not go higher with video cards unless you are going to upgrade the CPU in the next year or so. You can get a 7790 for pretty cheap here http://www.amazon.com/dp/B00BY3LTR0/ref=asc_df_B00BY3LT... if you go for a power supply upgrade, and the 7790 will keep up with a faster CPU at some point if you upgrade to an A10 or move to a faster system alltogether.