portable gps/bluetooth

Looks like i'll be getting another BMW as my daily driver, this time without their kludgey navigation system. Thinking about a portable system that i can use in all the cars. The ones specifically designed for auto use may,given perfunctory research, rely on an external antenna, which would seem to be a pain in the ass.
I'd prefer a large color screen, with integrated antenna, but don't know how that would track without an external antenna. Also, don't want to have to carry a bunch of memory cards; would prefer that the thing already have the entire US loaded into it.
Finally, doesn't seem like there is much in the way of Bluetooth compatability for these devices, yet. I was not really interested in a handheld PCA since i am not necessarily interested in address book, email and other features. What do you all know about this stuff and which is the one to buy? (I suppose software upgradeability may also be a factor, but i'm a little lost on this product area, so i can use concrete suggestions). Thanks.