The best is water but I don't think you have the budget for that.
I would suggest air as it will do the trick and is a bit more economical.
If you are looking for a huge, powerful, yet silent CPU cooler I would look at Zalman. Specifically the 7700-Cu. I can't recommend this thing enough. I get incredibly low temperatures with the fan going only 950 rpm and making absolutely no noise unless I open the case and put my ear right next to it.
I can also crank it up to 1800 rpm for better cooling and I still can't hear it. It's quite amazing. And because it is a 120mm fan it spills air over all your other components.
Here is a good place to get it:
The downsides are obviously it's large size and weight. It weighs about 2.2 pounds and makes your motherboard look like a piece of notebook paper.
Get a cooler like that with some Arctic Silver 5 and you're golden. On the site I linked to you can select it in the options box at the bottom for very cheap compared to newegg.
Oh and btw it fits very nicely on this board:
That is a very high quality board if you aren't planning on bothering with SLI. There is also a cheaper $118 version of that board that I use that works just fine. The 7700-Cu fits nicely for me but it obstructs the first 2 ram slots. This was fine for me because I just installed the ram first and then put in the heatsink. Or you could also just put the RAM in the 3rd and 4th slot.
Most motherboard work with it though, usually only the first 2 ram slot are blocked so you can't easily remove or put in ram there. I think it's worth the trade-off though. And of course there is also a smaller version of the Zalman CPU cooler.
Namely this one: