Depends more on how you use your available memory rather than how much memory you use. If you run a lot of load-time dependent intensive programs such as games, image editors, video editors, etc, you'll be better served by dual channel memory helping feed your processor plenty of data. If all you do is web browse, play Facebook games, and run it as a media PC single channel may be fine.
The computer is used 95%-99% of the time for the SETI@Home project, so just doing number crunching. I haven't played any games since upgrading to Windows 7 because of some video driver problems I have yet to resolve, and am unlikely to return to intensive gaming as my tastes have changed. I use Photoshop now and then, and burn mpg's to DVDs now and then, but no video editing and image editing at a minimum (cropping, scaling, adding text).
Can one even buy matched pairs of PC3200 memory nowadays?