The built-in VNC server is turned on in System Preferences' Sharing, in "Apple Remote Desktop" on Tiger and in "Screen Sharing" on Leopard and Snow Leopard. On Tiger, with ARD highlighted, also click on Advanced button to do VNC-specific setup. In Leopard and Snow Leopard, the built-in Screen Sharing application client is kinda hidden away, but you can launch it from the Finder menubar "Go" > "Connect To Server" and enter "vnc://yourVNCserverNameOrIPaddressGoesHere". You can configure Screen Sharing client to encrypt the entire session. Default from Cupertino, it only encrypts username and password. Not certain if, when you change it to encrypt the entire session, that preference persists for future sessions; you'd have to try it and see. I don't know what they use as Screen Sharing's encryption algorithm, as a total wild guess, I would say probably something like AES-192 like they do for FileVault. But I don't know.
Of course, if client and server are not on the same subnet (i.e., behind the same firewall) you will have to allow port 5900 traffic through the router firewall to pass to the vnc server computer, too.