I'm sorry if this has been beat to death but I couldn't find an answer by searching. I plan to buy a Garmin Etrex 20 for touring and hiking. Do I have to have the 100K base map? Is it suitable for navigation? Or can I use only the more specific 24K maps? I want to make sure I have what I need but not what I don't. Thanks!!

Kinda depends on what you want to do and where you want to do it.

Garmin-wise, out west and/or for off-roading, I'd prefer having the 24k maps. The 100k base map is pretty useless for other-than-main road driving/use and totally useless for hiking. The Cit Navigator North America map, on the other hand, is more than adequate for most of my non-off-road biking needs, if a bit pricey (@~$79-$90).

An alternative to the Garmin maps is OSM maps which are free.
Also check out www.GPSFileDepot.com/index.php

I've used both/all extensively in the La/Miss/Ala/Tx(central and eastern)/Florida panhandle areas and haven't found any reason to spring for the 24k Garmin maps (I have used a friend's copy/unit withGarmin maps for riding and hiking in this area when I only had OSM maps on my eTrex 20). The limited use I've had in the northern N Mex/So Colorado area while hiking reinforces my opinion - money no object>go Garmin 24k; money matters>go OSM/GPS File Depot.

The ONLY disadvantage I see with the OSM/GPS File Depot maps is currency; updates are irregular.