The "r" in GPSr stands for "receiver".

To cut to the chase.... all things being equal, I'd suggest the Oregon 450. If money is not an issue, then the 450t gives you the pre-loaded US Topo 100k maps and 4GB internal memory.
If you find a great deal on a 400t (I have one) then it's worth considering since it also has the preloaded map and 4GB of memory. But otherwise, I think the 450 makes the most sense. It has a slightly improved screen and much better compass, as compared to the 400t. The compass on the 450 series works in any position. On the 400t you need to hold the gps with the screen facing the sky for the compass to work.
To further confuse you... the Oregon 450 is the updated version of the Oregon 300, whereas the Oregon 450t is the update of the Oregon 400t. There is no such thing as an Oregon 400 (without the "t").
I don't think the 4GB internal memory on the "t models" is that big a plus honestly. You can put a 16GB micro SD card into the unit and have plenty of storage available. I would avoid the Oregon 200 because it doesn't have any internal memory. At this point, it's a problem because there appears to be a bug in BirdsEye which only lets you send imagery to internal memory and not a card. The 200 doesn't have the compass, altimeter or even a "beeper" like the other models.
For road navigation you will probably want City Navigator maps, which are extra. Garmin's 24k topo maps also have routable roads, but as maps4gps says - they are expensive.
I can confirm that the newest version of BaseCamp will let you use the pre-loaded 100k topo's from my Oregon 400t on the computer. They render a bit more slowly than maps that are stored on the computer, since they are accessed via the USB connection. They also cannot be printed: a dialog box comes up and says that printing is disabled due to copyright restrictions. Basecamp also lets me use the pre-loaded City Navigator maps from my Nuvi on the computer, with a similar limitation of no printing.
The BirdsEye aerial imagery looks great, but the service just launched a week ago and there are a lot of bugs to be worked out. If you have an Oregon and connect it to your computer, BaseCamp will allow you to download as much imagery as you like and use it on your computer for no cost. However, if you want to use it on your GPS you need to subscribe for $30/year. This seems really reasonable for an "all you can eat" plan. The subscription is only good for one GPS however. I plan to subscribe myself, but will wait until more of the bugs have been fixed before doing so.
True - you cannot use the National Geographic state series maps on any Garmin units. You should be able to transfer waypoints, routes and tracks (I assume), but not the maps. If you really want to use the National Geographic maps on a GPS, this is supported on the Magellan Triton series. I have a Triton 1500 but don't have any National Geographic maps. It's a nice piece of hardware, but not as nice the the Oregon series IMO. Magellan customer support gets consistently low marks from users. Fortunately, I have never had to deal with them myself.