There are very good soundfonts available at
http://hammersound.net/cgi-bin/soundlink.pl and they are free.
Pick ones that score over 8 and you should be happy with them.....and if you think they are good enough you could go for scores as low as 6 or 7 and still might find a few good instruments. I have found some of the drumkits fail to always play the drum note that they should. eg.....I can hear it if I click on it in the drum display in the graphic editor, but on playback it doesn't play.
If you have access to a PC (spit!) you can even download the ones that say "The SoundFont you are downloading has been compressed with the sfArk SoundFont compression program. If you haven't already installed this program on your computer, please download it from
www.melodymachine.com". PC only but expands/converts to .SF2 so they can work on a mac.
Never seen the mach 5 in action.
Absynth is quite amazing and well worth a look. It can create normal sounding organs and basses or really unreal outer space sounds.
(you can grab a OS9 demo........but I don't think a OSX demo is available).
Sampletank gives you a realistic set of instruments and has enough adjustment to keep you using it for a long time.
You could also just buy a CD of drums if you wanted......or pianos or basses.
The Drum Library from
http://www.fxpansion.com/ looks amazing but the manual suggests at least a 733Mhz Mac. No demo......but they do have mp3's of it to hear.
scoot.