^not really, no. 4gb is plenty for that. The extra 2gb isn't hurting anything, just not letting you run in dual channel. But the extra 2gb might as well be the same as 4gb dual channel for that use, it is unlikely that you would see any difference.
Hopefully he isn't using his computer for just Crysis, I'm guessing your dad is doing some kind of professional work? Crysis @ 2500x1600 resolution, Ultra High Settings and 64 bit doesn't even use 4GB. It is almost impossible for any gamer of any sort to use over 4GB, let alone 6.
God, I remember the day when Ram was everything. Now its the most overrated thing in the computer world.
I swear man, I work at Best Buy selling computers and what not. (I'm 17 so give me a break).. and I have to explain to 99% of people that come in what RAM is and why 2-3GB is enough for just browsing the web. Yet they still insist that the MUST have at least four because that's what their "geek friend" recommends...