As far as features/capabilities go, the Oregon series would be the obvious choice, but they lack the big, bright screen of the 276/478. The GPSMap 62s would be worth looking into as well.
I agree it would be nice if Garmin would come out with a true 276/478 replacement, but I don't think it's very likely. They seem to be more interested in selling different GPS units for different purposes rather than one that can do it all. (Oddly, the Oregon is largely an exception to that, aside from the screen size.)
The GPSMap 620 with the Oregon operating system/software on it would be a killer combination...