Consider the discontinued Bose Series II Headsets before you start down the soundproofing road.  I had these before swapping them for the new style lighter X model that my wife prefers.  The Series II are heavier due to the passive sound damping but are by far the quietest headset I have tried, comfortable too.  eBay is a source for them
http://cgi.ebay.com/ebaymotors/BOSE-Aviation-HEADSET-Series-II-Pilot-Avionics-Aircraft_W0QQcmdZViewItemQQcategoryZ26440QQihZ014QQitemZ330129608532QQrdZ1QQsspagenameZWDVW

As has been previously covered (see archives) minimizing wind leaks in the cockpit and directing engine exhaust away from the belly help.

Steve Colwell  Legacy in the Paint and Upholstery Shop