Storage

If you haven't yet, I'd highly recommend buying one of these reduced size MMC cards, so you can boot from MMC and have plenty of space for data.  This will also give you an easy way to revert to the untouched system.

1 gig
2 gig

GPS

You may also want to purchase a bluetooth GPS unit (I have instructions on this below):

Garmin GPS10 (sensor only) @ 118.62 shipped, the Deluo GPS @ 112.72 shipped, or the Deluo GPS with DVD @ 114.76.  I've heard good things about both of these, but for your own peace of mind, as they are somewhat expensive (although about the lowest prices you'll find for this quality) buying from Amazon makes returns relatively painless.  If you want a cheaper option, Geeks.com has some great prices on two units.

Connecting USB devices

And if you want to connect USB devices to the 770, like a keyboard or storage device (external drive, memory stick, etc), follow my directions here.

The Y-cable you need can be found with this 2.5" hard-drive enclosure.  At 9.12, shipped free, this is a great deal.