I recently got a Garmin Oregon 400t and I like it. The touch screen interface is a huge improvement over the other models, and the processor is noticeably faster (you can scroll and zoom the map quickly). The screen is much higher resolution than any of Garmin's other handhelds, so you see more map detail without zooming in as far.
The PN-40 looks like a very cool product, but the weak link is the screen which is actually rather low resolution. That's a shame considering the kind of imagery it can use. But otherwise I'm seeing good comments on it.
The Oregon is of course limited by the maps which it can use, which must be in Garmin's proprietary format. People have finally "cracked" that format however, and some homebrew maps are floating around now, but I have yet to find a topo of New Jersey. The Oregon also has some new features which their older units did not have, and it seems capable of displaying images (satellite photos, scanned maps, etc) as well as traditional Garmin vector-based maps. But nobody has cracked this new format yet to provide us with tools to make that kind of custom map. But I think it's coming.
The Oregon screen leaves a bit to be desired in bright sunlight. It's readable, but could be more vivid. Evidently this is due to the plastic touch screen and non-reflective finish. I don't find this to be a big problem, but would be happier if it were better.
Yes, you can install the Garmin topo maps on any of the Nuvi's. Just buy the DVD version of US Topo 2008 and use mapsource (or RoadTrip on the Mac) to install whatever areas you like on a memory card (I'm assuming your Nuvi has a card slot). Alternately, you can install them directly to the Nuvi's internal storage, but will be limited by the amount of free space available.
After doing this, on the Nuvi you tap Menu > Tools > Settings > Map > Map Settings and you will see a listing for the builtin maps (City Navigator) as well as the Topo Maps. Check the box next to the Topo's, and uncheck the box next to City Navigator. You'll now see the topo maps.
Note that they are not "routeable" however, and can't give you driving directions. You will also have far fewer menu/configuration options on the Nuvi as compared to a "real" handheld GPS. But the maps will work fine, I use them all the time myself on a Nuvi.
You could also buy the storage cards with pre-loaded maps, but that's not a very good deal. Costs about $80 for just a couple of states, vs about $100 for the entire US on DVD. Plus, with the DVD version you also get the maps to use on your computer, and can load them on more than one GPS.
Garmin has a new series of 24k topo maps which they are gradually releasing for each state. These look like a nice improvement over the regular US Topo which is 100k resolution. They also have fully routable roads and tails. Here's a review:
http://gpsinformation.info/penrod/24Kmaps/twofourmaps.html. Unfortunately, they aren't yet available for New Jersey....
Here are my favorite forums for info on Garmin GPS'es:
http://www.gpsreview.net/forums/viewforum.php?f=21
http://www.gpsreview.net/forums/viewforum.php?f=2
http://www.gpspassion.com/forumsen/forum.asp?FORUM_ID=172
http://www.gpspassion.com/forumsen/forum.asp?FORUM_ID=4
http://www.gpsinformation.org/forum/viewforum.php?f=22
http://www.gpsinformation.org/forum/viewforum.php?f=4
http://forums.groundspeak.com/GC/index.php?showforum=11