Romer that depends on what you mean by "topo maps". While the high end GPS units will do a topo map, they are not very detailed. For 4 wheeling they are probably accurate enough, but I am used to orienteering with the USGS 7.5" topos, and no GPS has that kind of detail. So if you want to find something large, like a mountain or road, they are adequate. If you are trying to navigate in the backcountry I strongly suggest you use the lat/long feature of a GPS in conjunction with a good topo map.
I bought he Vista C I think? It was the nicest color unit I could buy at REI, and I took it back. Maybe I have been a map and compass guy for too long.