i think you should go with the Opteron 165, even if u are just going to 2.5GHz, because 1st of all the Opteron will get there much faster, especially if ur getting PC 4000 Ram, even without it, it should be a breeze, and the 3800 might take u a while to get to 2.5GHz, and on top of that, when ur computer is gonna need to meet higher demands, you can OC that Opteron even further, with air you can get it to 2.8GHz, and with water cooling you can get it to 3.0GHz, which you cant do at all with a 3800, + on top of that it has half the cache of the Opteron.