PHYSICSGUY  You shouldn't have much free memory. What you want to make sure of is that your free + inactive memory isn't low.

The reason people see slowdowns when they run this program is that you're effectively deleting a bunch of cached data (like Safari tabs you aren't currently viewing) from memory, so you have to reload it from disk.

Here's an Apple document that goes over the types of memory you see in Activity Monitor and what they mean: http://docs.info.apple.com/article.html?artnum=107918  
(Version 2.0)