Also note that OS X dynamically allocates virtual memory. From the command line, 'ls -l /var/vm' (or 'ls -lh /var/vm' if you're not too familiar with the Unix command line and want it more readable.) You can see the swapfiles there. You start with one 64MB swapfile, and more are created as needed. A reboot will clear them out.

Of course rebooting is silly; it's better to just not end up with huge swapfiles. I've found the worst offender for memory usage is Safari. I find that if I restart Safari at least once a day (and empty its cache), it doesn't grow to huge proportions.

You can check the actual memory usage of your applications with 'top -o rprvt'. The number under the RPRVT column is the memory usage. Expect Safari to be way up there if it's been running for a while; I've had it grow past 500MB before. Horribly, leaky application (but otherwise a great browser!) If anything else is a huge memory hog you'll see it in there.