As far as I know, placing the program (not the maps) on the main memory of your pda, is quicker dan placing it on a memory card..
But I've got TomTom, and I placed it at first @ the main memory, but now the program and the maps are placed on my sd card, and I didn't notice any difference..
But Navigon is kinda slow, so maybe the best thing to do is to place it in the main mem.
Just test it..
Sorry for the not so good english..