I've been using an Android app called GPS Essentials on my phone. It's one of the few out there that has options for MGRS for coordinates and Mils for bearings, both of which I use at work. It basically does everything a handheld GPS can do as far as waypoints, tracks, etc. Map wise, there are a few options, but I usually just use google's satellite or topo on it - provided I have cell reception that is. That's the only catch with not having a preloaded map. But, I also have a little Garmin hand held that I use most of the time when I'm on foot anyway.
I'm suprised that you can't put Garmin's topo on a 430n though, since it basically uses the same nav program as their other dash mounted GPS units, and I think they can all run the topo maps fine. I may have to do some playing with mine to see if there is a way to transfer it over and get it to display... right after I get a new topo disk anyway, the one I have is a couple years old.