Most of the Garmin NUVI units can display topo maps - you can go on the Garmin web site and pull up the unit you might be interested in and it will tell you if topo maps are an option. But they don't come with topo maps - you'll have to load the maps onto an SD or mini-SD card and add them to the NUVI that way. You can buy the topo maps from Garmin or a variety of other sources - or you can download them from GPSFileDepot.com. I've found them to be in most ways superior to Garmin's offerings. Especially for Nevada, the Garmin 24k maps don't have a lot of the dirt roads on them; generally, they will be on the GPSFileDepot maps.
I also have the Garmin NUVI 5000 - it is a wonderful unit with a big, bright, sharp screen. But like any other NUVI, a lot of the features available on your handheld are not available on any NUVI. That's just the way Garmin sets them up. For instance, when I am going to explore an area new to me I bring up the area in question on the map on the computer and, using the Track tool, highlight the road/trail in pink. I then load that track onto my Montana and when I get to the area all I have to do is follow that pink line to follow the roads I am interested in. You cannot do that with a NUVI. As far as I know, no NUVI will allow tracks to be loaded to it.
So, if you are looking for a big-screen GPS that will do all that your current unit does, you cannot use a NUVI. You'll have to go with something like the Montana which can function as both street navigator and trail navigator.