Folks, stay calm
The question was Opty or FX60. Given the initial posts (Opty 185 or FX60), the budget is clearly in the $400 - $450 range.
Regardless of fanboyism or being a 'green' guy or 'blue' guy, at those prices, the Core 2 Duo is a serious alternative. A good set up with CPU, MoBo, and 2GB RAM, should cost no more than like $600. With his current cash, and selling the old MoBo/RAM, that's easily afforable. (can you say below-20 second superpi 1MB?)
However, a ton of money can be saved by getting an Opty 165 or an X2 4800. That would be really good upgrades.
Dropping an $400-$500 CPU in an outdated technology rig (socket 939) is not the thing to do in my opinion. However, $150-$250 is a very viable upgrade, and will give him a powerful rig.