i have a opteron 165-the heatsink is very sizeable and weighs nearly as much as my zalman7000B-cu plus it has 4 heatpipes it is an excellent processor get it or the 3800X2, i used my bro's pc which has one and it changed my mind and i went with dual core-this processor is also good and will save you a little money over the opty
and this heatsink is much larger and a better cooler than my bro's 3800X2 heatsink
a zalman 7000B-cu is very quiet and very good cooler (i have one)
good PSU is very important don't skimp on it
7900gt is an excellent video card for the price, in some benchmarks it compares to that of a 7800gtx
an asus A8N-SLI mobo is great quality and u have the ability to uprade but if u don't want it the epox one mentioned is good and so is the asus A8N-E