Since you're not overclocking, any single tower cooler with a 120mm fan will be more than enough, and be a magnitude quieter than your stock solution.
The Xigmatek Gaia is probably the cheapest solution that is of a good quality. It is usually around $30 bucks, though if you are lucky you can snag one for $25. If you are not looking for anything special, I think this would probably be the best investment for you, and I believe this for a couple reasons: not only if it the most affordable, but, out of the popularly recommended solutions, it is generally accepted to have the quietest fan at full tilt - although that's splitting hairs.
The Cooler Master Hyper 212 EVO is probably the most recommended solution on this site. For its price (~35) it allows for people to pull off decent overclocks. It is generally accepted as the price-performance leader (although others dispute that) and, for overclockers, it is not worth it to invest in any other solution until you reach the $60-90 dollar price bracket, where actual appreciable gains can be had. (In the world of aftermarket cooling solutions, there is definitely a strong element of diminishing returns. The difference between the CM Hyper 212 EVO and a high-end dual-tower cooling solution can be less than 10C.) Since you do not plan on overclocking, some people will recommend you get the Hyper 212 Plus instead, since it generally is cheaper, although its performance is slightly less.
The Enermax ETS-T40 is another good choice. The basic model also runs around ~$35. It is a very good quality solution that is quite efficient, thanks to a number of innovations it has with its design of airflow. Its only downfall is the design of the base is of the first generation of Direct Touch heatpipes, so when it comes to extreme/enthusiast overclocking it can struggle to keep up.
Though more expensive, the Thermalright True Spirit 120M and 140 might also be good choices. The 140 will probably be the quietest, but you'll need at least 170mm of clearance to fit it into your case.
I used to recommend the Arctic Cooling A30 Freezer, since it was seriously a quiet solution, but I no longer feel it is worth its cost. (It only allows for only one fan, and comes with a very limited set of mounting equipment. A30 is for AMD, I30 is for Intel.) If you can get lucky and can snag one for $30, sure, but there are better heatsinks out there. Instead, I'll recommend people trying Actic's F12 PWM fan on their a heatsink of their choice instead. The fans are only $10 a piece, so its not a huge investment. (A CM Hyper 212 EVO + Arctic F12 fan = $45. The same as what the A30 Freezer is on sale for right now. They will probably get similar performance, but I can throw another fan on the EVO, so what is the point?)
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