I'd like to make some improvements to my PC to make it run quieter. I'll outline what I have and what I'm thinking of doing, and hopefully I can get some feedback from you.

Current build:
-Case is Rosewill Challenger (Not a big fan of this case, just got it because it was cheap and I thought the case didn't really matter)
-CPU is i5 2500K. Stock fan.
-Case fans are all stock.

I've done some testing and the CPU and case fans are the largest source of noise.

Here are my ideas:
-Buy a new case? I've heard good things about cases designed to be silent, such as the Fractal Design R3/R4.
-And/or, Buy new CPU fan/case fans? I was looking at the Cougar Vortex fans. I'd need 3 for the case. Then, any suggestions for a simple quiet CPU cooler?

What could I do for around $150?

Thanks!

hyper 212, plus or evo model work great


as to fans, Im a fan of noctua (pun intended)
2 p or f12$ would cover you

as to cases fractal design r4
looks amazing as per reviews

I see that the Rosewill Challenger has an open mesh front and then side/top holes as well, so it's a very "leaky" case as far as noise goes. Getting a case like the Define R4 would make a big difference, and running the CPU at standard speed might even make the stock CPU cooler acceptable inside a good case. But a relatively inexpensive cooler like the Hyper 212 Evo will certainly be worthwhile. You wouldn't need additional fans for the Define R4, but perhaps one of the old ones could fit with the fan controller in the case.

I just swapped to a Define R4 from a Lian Li Tyr PC-X2000. The Lian Li had 4x 140mm fans, the Define R4 is cooler or the same temperatures with just the 2 stock 140mm fans running at 5v. I'm fairly certain the only thing making noise in my case now is the Tagan PSU I bought 4 years, which I think I'm going to replace soon with a lower noise one. I had to mount my single 160gb WD HDD from 2007 with elastic though, it vibrates far too much and it drove me mad in about 30 minutes using the standard HDD mounts. You can't go wrong with the R4, though.

Can vouch for the Hyper 212 as well, running at 10% speed my i5 2500k @ 4ghz is at 35-40 degrees idle and doesn't break 50 degrees during load, even with the fan running at 30-40% speed.

Even a quiet case can't eliminate all the noise from your current CPU cooler and fans. So I'd advise you to change these components first. If your computer is still too loud, then you can consider buying a new case.

The Cooler Master Hyper 212 EVO is a good cooler. The more expensive Scythe Mugen 3 is also worth the money. For case fans, I can recommend any of the Scythe Slipstreams and Nexus Real Silent fans. These are well-regarded by the SPCR forum and they are cheap too. They do need to be undervolted though (unless you go for 500 rpm or 800 rpm Scythe fans).

Other sources of noise are the hard drive, the PSU and the video card. It takes a bit of effort to quieten the PSU and the video card, but if your hard drive vibrates a lot, a simple solution to eliminate the vibrational noise is simply to suspend it using elastic cords. There are aftermarket solutions to eliminate all vibrational noise. These can be had for ~20 USD, at least in Europe. I can't find any internet retailers that sell these products in the US.

As a side note: It's definitely possible to have a quiet computer evne though the case is "leaky". My Cooler Master 341 does nothing to prevent noise leakage but my components are so quiet that the case doesn't have any impact on the overall noise level.