If you go for the 7750, I advice you to get a PSU with more W, as the recommended minimum is 400W (as stated on AMD's site). My recommendation: COOLMAX ZX Series ZX-500 at 39.99$ - the cheapets 80 plus certified PSU between 451- and 500W.
Why not go for the RADEON HD 7850 instead? Its performance more than justifies the price difference between it and the 7750. Since a bad PSU can make your system slugish and/ or result in shutdowns, I would also recommend a certified PSU to ensure the system is running well. One cheap, certified PSU is the
CORSAIR Builder Series CX600 at 59.99$ - if you want to stay safe and have solid performance, you
will have to pay
This is a very good deal with the 10$ discount(original price is 69.99$). Note that you won't need this kind of power if you go for the 7750.