What Digger said:
Safari -> Preferences -. General tab -> "Default Web Browser" selectbar.
Edit, though I would advise against Netscape as a default browser. It's a bit bloated and out of date. Netscape 8 has been out for a LONG time now and still no OS X version, so 7.2 is languioshing further and further behind.
So, I would suggest either sticking with Safari (there's not much wrong with that), or trying Firefox, which is Netscape-like, but less bloated (for now).