Good but cheap Graphics card for HP Pavilion a6637c?

I want to play some games on my pc such as Ths Sims 3. My graphics card sucks in my HP Pavilion a6637c. I was told If I got a replacement it would solve the freezing problems and the games garbage graphics on my pc. What wouls be a good but cheap graphics car for this? Under $100 or even $50 if it is possible.
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