For about three years for Mac-based paperless geocaching, I have used a PowerBook G4 1.67GHz (earlier PB G4 1GHz), and a Palm Tungsten T2 PDA (earlier Tungsten T1). And for GPS receivers, we currently use/have used a Magellan Meridian Color, Magellan eXplorist 400, Magellan Meridian GPS, Magellan Map 330, and Garmin eTrex Legend, with two PB G4's.
We download a gc.com Pocket Query with caches of interest, and use MacCMConvert to convert the .GPX file to a .PDB file for CacheMate on the Palm, and use the basic Palm Desktop HotSync feature to load the PDB file to the Palm (by cable or bluetooth).
We upload the Pocket Query .GPX file to the GPS receivers using MacSimpleGPS, or MacGPSBabel, via the standard Magellan or Garmin serial cable with a Serial-to-USB adapter. We are now experimenting with the latest Mac OS X version of GPSBabel. And we just learned about MacCaching for OS X, so we will give that a run too.
I have occasionally run GSAK on VirtualPC on the PowerBooks, but personally do not find enough features/advantages to GSAK to offset having to use VirtualPC, and last I checked GSAK still did not have USB support for Magellan.
If you have questions, I will be happy to help with my experiences from using this setup.
Links to software we use...
MacCMConvert
http://jfatherton.0catch.com/MacCMConvert/index.html
CacheMate
http://www.smittyware.com/palm/cachemate/
Mac SimpleGPS
http://www.macsimplegps.com/oldsite/index.html
GPSBabel
http://www.gpsbabel.org/
MacCaching
http://www.maccaching.com/
Geocaching Swiss Army Knife
http://www.gsak.net/