It's .aiff in X. I used SoundHack
to convert some of my old sounds to .aiff. You may have to fiddle a bit with some files to get them to work as system alert sounds.
Any installed default OS X alert (in /System/Library/Sounds) is an 'AIFF Audio File'. My converted sounds are 'AIFC Audio' and both types have the .aiff extension.
I placed my converted sounds in ~/ME/Library/Sounds. Once I got them in the correct format (that's the fiddling in SoundHack) they worked fine as OS alerts and in most apps as well--but not in iCal. iCal only sees the 'official' alerts in /System/Library/Sounds. Well, that's just one of iCal's problems, I suppose.