Caldrumr :
Well, the fact remains that you will see little performance difference today. In the future, I am sure you will. However, when people were making those arguments about quad cores, it was probably true. Many applications today are still not making efficient use of four cores. Some are, and some will make use of six cores. But they are the types of applications the OP is stating they will not be using. So, unless they plan on keeping their CPU for a very long time, they will replace it long before they will see much difference. That's all I meant.
The X6 is 20% faster at the same clock speed. I am unaware of a single benchmark it loses to an X4 at the same clock. That's significant. And it doesn't even count peak output. Which becomes valuable over time.
It doesn't matter whether a particular app uses multiple cores. Take a look at any machine. The number of threads running is staggering. Antivirus, trey programs of all types, OS tasks, and that is before you double click anything.
Over time application and OS loads tend to increase, making the extra raw power very valuable few years down the line. Thats why 4 year old quads hold up well while 4 year old duals that were considered on par with said quad has not fared so well. That trend will continue for the foreseeable future.
Even if he doesn't use the system more than a few years, it will likely be passed down to a friend or family or used as a second machine.
So in my opinion, dropping an extra $30 to $50 is a worthwhile investment.