New sounds for OS X

I've been looking for sound files to add to the slim selection included with X. I've read somewhere that you ned aiff files and you can just place them in the appropriate spot in the system and you're golden.

Now, I've only come across the old sound sets (god I miss the star trek set and the HAL one...) from pre-X. Anyone know where I could get some cool system sounds?

Also, where exactly would i need to place the files for the sound pane to see them?

Thanks in advance guys.

Side note:
do you see any reason why we couldn't have the same sort of SoundSet setup like before? It seemed like Apple went out of their way to provide this functionality and then dropped the ball (leaving only 'Platinum'). Fortunately 3rd party developers picked up that ball. Could this come to pass again???
There are two places to for the sounds file /System/library/Sounds and ~/library/Sounds (~ means home dir). I put my sounds in my user's library/sounds dir. The sounds are in AIFF format with the aiff file extension. Without the extension the control-panel does not 'see' the sound. I got my sounds from and converted them with SoundApp.

You will also need to restart any applications that need to be able to see the new sounds. It would be nice if they could automatically update their list on the fly.
Thanks. I went to clixsounds too but could not for the life of me remember "soundapp"! to convert the files to aiffs. It's not X-ified or is it???

Do you know of any sound converters that ARE X compliant. I detest going back into classic unless I really need to.

On a related note: does anyone know the rationale behind using the aiff format? Just more widely used or what?
The version of SoundApp that I am using is a classic app. I have no idea whether there are any native apps that do the same job. In the meantime SoundApp allows you to convert the sounds at ClixSounds into a format that is useable by OS X.

If anyone finds an OS X app that does the equivalent to SoundApp then I would love to know.