Juan,
It is good that you are checking out a new PSU (power supply unit) because the majority of the PSU's that best buy uses are very cheap and of low quality. Anyone who is an expert on building PC's will tell you that having a high quality PSU is VERY important! For if the cheap one fails it can kill all the rest of the components in your computer...not good at all. Not to mention you won't get the maximum performance out of your other components. Anything over 400W will be good for you. 600W would be great if you plan on building a much more powerful computer in the future, but it is unnecessary if you plan on sticking with a low power graphics card like the Radeon HD7750.
I always recommend Seasonic, here is a good one for you
Seasonic 430W for $59.99 + Free Shipping -
http://www.newegg.com/Product/Product.aspx?Item=N82E16817151074
Very high quality - will outlast the rest of the components in your computer. Also, with power consumption consistently dropping lower every generation, I have a good feeling that within a few years a quality 430W PSU will be able to run a top of the line single card system, so it should be rather future proof as well
Good luck.