I just upgraded from an eTrex to a GPSMAP 76CSx and had the same question (before I found this forum).
Yes, the maps work. POIs, maps, streets, finds, etc ... all there.
But. the new auto-routing feature that you'll have on the 76 doesn't work right in the actual GPSr (on the PC it still works of course). It will come up with a track to follow that will be way off, by-passing obvious roads, etc
For that reason (and I was on Metroguide 4 circa 2002)I upgraded to City Select. Map functions and details pretty much the same, but much better auto-routing.