Earlier quoted context omitted.
Open activity monitor, and check which applications use a lot of memory. (Look at the "Real Mem" column). If your computer frequently freezes, the problem is mostly a specific application (Virtualbox comes to mind), not just Mac OS X itself. 4GB are more than enough for casual browsing.
Activity Monitor is a tool too blunt for these kind of scenarios. It pains me, Mac OS X has DTrace which makes it a breeze to find out what really happens, however, the only GUI tools built on top of DTrace are profiler for programmers, nothing for the casual user. Activity Monitor presents data that's not really useful and does so in an intrusive manner and in a confusing display. Mac OS X already comes with parts o…
If you want to know why a specific app is frozen (beachballing), you can click the "Sample Process" button in Activity monitor to perform a time profile.
Yes, if you want more details, you have to use Instruments or write your own DTrace scripts. But I doubt that even a time profile is useful to the "casual user".