I highly recommend getting a large MMC card, so that you can boot from it (see Booting from MMC). Currently, 1 or 2 gb are reasonably priced and offer plenty of space for all of the fun stuff that can be done with the 770.
I also recommend getting a bluetooth GPS unit. The 770 has a free, open-source application called Maemo Mapper that is one of the finest GPS/routing packages I have ever seen (commercial or not), and it's only getting better.