I've recently started caching myself after buying a handheld GPSr (a 60csx in fact). The 60csx is a great receiver.
I bought the unit primarily for use while walking in unfamiliar cities both in the US and Europe so I purchased the Metroguide versions for North America and Europe off of eBay. Both products provide detailed maps and POIs for their areas. They do not, however, provide routing capabilities on the 60csx (only on the PC) without using some hacks so aren't that useful for auto routing. Since I already have a GPSr that does auto routing (see more info below) for the US and Canada, I didn't opt for the City Navigator (CN) version of the maps because a) they were more expensive, I rarely drive when in Europe, and c) they are locked to the receivers and I wasn't sure I wanted to try to sell a unit with the European maps down the road. Perhaps, some day I'll pop for the CN products but I'm happy with the maps I have for now.
While reading the manual I came across the geocaching feature which sparked a few brain cells into remembering that I had heard about it before but had never pursued it. Upon investigating, I found out that there's a pretty good density of caches in the Chicagoland area so figured I'd use my new toy to get off my butt and get some exercise/fresh air. I also like solving puzzles and similar mental agility type things so this is a pretty good fit for my personality. So far I've been having a blast. It'll also give me something to do when I have some free time when out of town.
As far as using it in the car, you'd want to go with the City Navigator North America NT version. The CN products provide routing within the GPSr and the NT version will fit the entire map set for North America on a 1GB micro SD card with some room to spare. Of course, the non-NT version will all fit on a 2GB card. If the price difference between the NT and non-NT version of the maps is significant, and if you don't need all of North America at once, you could opt for the non-NT version; the data is the same on both versions, just compressed differently.
My Metroguide shows quite a bit of detail for Vancouver and I'm pretty sure the CN product has the same level of detail (plus routing info for the GPSr). You can check what each Mapsource product contains via the Mapsource viewer tool on Garmin's website (http://www.garmin.com/cartography/).
Since I have another GPSr (Lowrance iWay 500c) just for use in the car, I find the handheld to be less than optimal for auto use. The interface is a little more cumbersome due to the lack of a touch screen which makes data entry much easier. Also, the smaller screen on the 60csx, at least for me, it too hard to see from the places I'd want to mount the receiver. My 500c sits on the dash where I can easily reach it and, even more importantly, see it without having to move my eyes too far (less movement required than looking at the speedometer). However, I got used to the larger, car-only unit first so your mileage may vary.