I second the Zalman. I use it on an AMD 2,200+ (=1,800) with success.
The DAW has
- Enermax 431W controllable PSU, fan set to quietest.
- Zalman CPU and nortbridge cooler with stock fan set to slowest.
- Matrox video card without cooler.
... and it is basically silent.
This thing is together with a non-quietised PC (AMD1,400) inside a homemade isobox (with 2 80mm fans intake and 1 outtake, set to 6V for quiet operation).
When both machines are running you can hear a faint sound, when the DAW is the only machine running you won't hear it.
Regarding temperatures: both machines never reach 50°C according to the BIOS when operating a full day.
Things that could've been better:
- special quiet fans for in and out of the isobox. The ones inside are spares from some old computer cases.
- quiet 120mm fan to replace stock Zalman one, it'll move more air for even less noise.
- the isobox was kind of quickly put together from 18mm MDF and some 2" foam. Design and construction are not flawless (service lid at the back, real box-in-a-box construction, leaking, glas door with rubber seals,...), but then again it cost like 50$ in total.