When you're fed up with your Mac telling you it's not about to open the pod bay doors and wondering if, Dave, you really want to do that, you may want to think about making some custom sounds for it yourself. And, as luck would have it, there's a great tutorial on how to do just this over at Macworld.
The article looks at both recording your own sounds and using GarageBand to convert your sound files, and how to add the resulting sounds to your system.
The article walks you through capturing your own sounds using a microphone and QuickTime Player in Snow Leopard, including how to get rid of initial pauses and the sound of you clicking your mouse at the end of the recording.
And it goes on to discuss how to convert your recordings to the AIFF format needed for system sounds in OS X using iTunes or GarageBand, and then how to insert the sounds into your system and attach them to specific events.