Well you have a pretty decent range of cpu coolers. Obviously, best bang for your buck would be a cheaper model such as the hyper 212+ which goes for about $30 though you can find it on sale occasionally for $20 or so. It is pretty much diminishing returns after that. Top of the line for you at about $80 would be the noctua nh-d14. It is a massive dual tower cooler with support for 3 fans. Problems with bigger towers like this and even big single towers is making sure it fits in your case as well as clearance for large heatsinks on ram. If you have low profile ram, you would be fine. The nh-d14 also has very low noise for the performance, so if noise is an issue, that is another thing to consider.
A couple of good $50 or so coolers are the thermaltake frio and this:
[ame]http://www.amazon.com/Spirit-passed-Intels-testing-officially/dp/B00629G1PG/ref=sr_1_13?ie=UTF8&qid=1341168229&sr=8-13&keywords=thermalright+cpu+cooler[/ame]
Here is the smaller 120mm version that is $10 less
[ame]http://www.amazon.com/Thermalright-100700529-True-Spirit-120/dp/B005MSOH7C/ref=sr_1_9?ie=UTF8&qid=1341168229&sr=8-9&keywords=thermalright+cpu+cooler[/ame]
Unfortunately, looks like prices went up a couple months back on both by $10 which makes them slightly less attractive but still good deals.
Another consideration is the type of overclock you are looking for. I would like to know what type of temperatures you are pushing on the stock heatsink at 3.2 ghz 24/7. Temps seem like they could be quite high unless you undervolted quite a bit. But, obviously, if you are looking for a mild overclock like you have now with simply reduced thermals, then a 212+ would be a vast improvement and plenty. As the thermal output increases with a higher overclock, that is when the more expensive and better coolers start to distance themselves from the pack. Thus there are quite a few things to consider.
Lastly, there are some self contained liquid cooling systems from antec and corsair that have very small units that attach to the cpu which is nice. However, they have a fairly large radiator with a 120mm fan that needs to be mounted to the case for exhausting the hot air from the cpu. They are comparable to some of the air coolers, though they usually draw a price premium for similar performance. They are fairly noisy compared to most cpu coolers due to the high rpm radiator fans that come with them. Just another option if you are interested in it:
[ame]http://www.amazon.com/Antec-Kuhler-H2O-620-Liquid/dp/B004LWYE4Q/ref=sr_1_1?s=electronics&ie=UTF8&qid=1341168718&sr=1-1&keywords=antec+kuhler+620[/ame]
