None. Powerfull and Quiet do not mix - Powerfull emits lots of heat which means lots of cooling. Lots of small cooling = lots of airflow = noise, as sad as it is.
The best options are:
* Get a low power laptop, use RDP to a powerfull workstation / server.
Alternatively:
* Wait a little - the Ivy Bridge are now pouring into the laptops. As they are twice as efficient as Sandy Bridge (half power usage, about)... which means less heat to move and less power usage (less drain on batteries).
The WIndows 8 "Generation" of hardware will be a big push coming in Q3 / Q4 this year. I am not saying "Windows 8" but with the Windows 8 focus everyone is aligning new hardware releases, and this time they have a really good BIG step forward lined up.
On another note - depending how you define "Powerfull" the AlienWare machines are about he most powerfull as laptop you can get. You pay a steep price though... Price, Battery Range and - under load - NOISE.