It can be done, it's simple, all you need is this application:
http://namedfork.net/bootxchanger/
Edit: And for the gong, you cannot change the noise, but you can turn it off or make it softer with this pref pane: http://www5e.biglobe.ne.jp/~arcana/StartupSound/BETA/index.en.html
Follow these instructions:
http://paulstamatiou.com/2007/10/31/how-to-change-leopards-login-wallpaper
Boot image not the login wallpaper.
RTFT
It can be done, it's simple, all you need is this application:
http://namedfork.net/bootxchanger/