o crud, I ran out of time....
Ok, for the E6400
Package Type: LGA 775
Thermal Design Power: 65W
Thermal Specification: 61.4°C
Core Voltage: 0.850V-1.3525V
You may wind up undervolting to lower your power dissipation.
Airflow through your case is important consideration, so check out the following link for ideas .... Thermally Advantaged Chassishttp://www.intel.com/cd/channel/reseller/asmo-na/eng/53211.htm
Get a good case. Then think about getting an 945 (Intel nomenclature) Chipset based mobo, e.g. Gigabyte GA-945P-S3 or GA-945PL-S3 or Epox EP-945 Pro. Get a good heatsink, e.g. Zalman CNPS 9500 or Scythe Infinity with a Nexus 120 fan (or better). Run without the fan -- but watch the CPU temps closesly -- or run with the fan at the lowest RPM. Use Sound Dampening materials in your case to quiet the fan, if you run it. Check out http://www.silentpcreview.com/
for more ideas along those lines.
I'm outta time. Gotta run.
Btw, how are you using the computer? Is it for an entertainment PC, or a desktop, Enterprise desktop, mobile, small business, etc. ?