Topo! 3.4.3 only has the ability to make a serial connection to a GPS, and while Topo! (BRE) 4.1.3 does have a choice for "Garmin USB" it won't make a connection to my GPSmap 60C. I use a Garmin serial cable with a Keyspan <http://www.keyspan.com> USB to Serial adapter (I have a Keyspan USA-19QW) successfully with my Macs, PCs and Virtual PC 6 and 7 running on Mac OS X, with various software packages with my Garmin GPSmap60C and eTrex Legend. With the Keyspan adapter and a serial cable, I can also run Garmin MapSource under Virtual PC. (Garmin MapSource doesn't work with USB under Virtual PC; check this URL for more:
The advantage of a serial cable is that it is required to transfer data via NMEA protocol. The advantage of a USB cable is that it is much faster for transferring MapSource maps (otherwise it's probably not a big difference).
Garmin says their software will work under Mac OS X this year <http://www.garmin.com/pressroom/corporate/011006.html>, and the Topo! software is in a state of flux right now (major changes to the Mac version in progress), but if you want it now I recommend the serial adapter and cable solution. It just works.