Do you have a PSU? What is your budget for both?
If you are using the stock HSF form AMD, then the largest impact to your noise levels will be from your video card fan, PSU fans and then case fans. On my rig I went from a Sapphire X800GTO to an HiS X800GTO and the noise coming out of my rig went from a loud hum to near silent.
Having used the Antec Sonata II for a few months now, I love how quiet the case is. I'm still using the stock Antec Smartpower PSU and that is the next place that I could reduce some noise - but not a lot. I would recommend the Sonata II - especially if you can find a rebate deal.
My next build will be in a
Coolermaster RC-531 and have a
Fortron 450W. The PSU is good/reliable brand and notice the large exhaust holes and the 12cm fan for reducing noise levels.