Re: card for sat-nav.
I am no expert, but from my experience, the faster the card the better, and there is plenty of discussion on this site regarding the size of cards. From others experience, it appears that as the card gets bigger, the stability is reduced, therefore, a higher speed card is needed.
Regarding what size is needed, I used a 128meg card for 2 years with tomtom, I now have a 256meg with plenty of spare storage. Given curent prices, I have just ordered a 1gig fast card. I will update if it falls over!!