which jam pack contains voices?

I can't say for sure, I think it was JP1 that had a choir. Check these out, I believe they are the correct URLs (if they didn't change them on me again) that list what instruments are included in each pack:

JamPack 1
http://manuals.info.apple.com/en/GarageBand_JamPackTips.pdf

JamPack 2
http://manuals.info.apple.com/en/GarageBand_RemixToolsTips.pdf

JamPack 3
http://manuals.info.apple.com/en/GarageBand_RhythmSectionTips.pdf

JamPack 4
http://manuals.info.apple.com/en/SymphonyOrchestra_InstrumentTips.pdf

--HangTime [Will Compute for Food] %-|>

See the "Good Choir Soundfont?" topic thread by Igby. Check out my reply (Spider Closet Studios). I have GarageBand Jam Pack 1, Version 1.0.1, and I haven't found any voices or choirs in it.

You most definitely have the system requirements.

As an aside, would anyone know which presets are offered in the Jam Packs, especially those designed for vocals? Does anyone think there are some really good presets in the Jam Packs?

The M-Tron reference is to MINIMUM system requirements. That means the M-Tron plug-in will run on a minimum of PowerMac 604e 200MHz 128MB RAM or BETTER. The iMac PowerPC G5 1.8GHz is definitely better.

After further exploration, I discovered that Jam Pack 1 does indeed have 4 choir emulations, all of them synthesized and electronic sounding, but nonetheless pretty cool for space music. The 4 choirs in Jam Pack 1 can be found by clicking on the New Track button (the rectangular gray button with the plus sign on it below the tracks). This opens the New Track window. Select Software Instrument, then use the pull down menu arrows to select Jam Pack 1. In the box below the menu you will find:

Choir

Alien Tempo (soft wah-wah voices with echo)
Basking Glow (crescendo voices with sustain and echo)
Mixed Choir (electronic aah)
Olympus Voices (electronic aah with phasing/flanging)

All are apparently synthesized electronic voices. If you prefer real human voices over electronic, the EastWest Symphonic Choirs and the M-Tron (Mellotron) choirs mentioned in a previous post are from real human voice choirs.

Ah, so I did remember correctly. Cool.