OK, I few notes (I have a Sprint 8703e, came from the 7100T (T-mobile), and have pocketPC experience in the Dell Axim and HP PPCs):
I do not use the GPS feature, although I think it is cool. I believe I did see some compatible mapping software (and others here know it better) that you can purchase from handango as opposed to Telenav. Spot for BlackBerry claims to be what you want for about $50.
Spot for BlackBerry by Skylab Mobilesystems Ltd. - Handango BlackBerry 8703e Software
I have used my 8703e to tether to a laptop. The two times I have used it other than just for testing, it was great (because I would have been without the internet otherwise). It seemed slow at first. but then did OK. I would never go without home internet in favor of using this, unless times got real tough or you are a very casual user who mostly checks web based email and a few sites that were not heavily graphics intensive. (I would not dump the home internet service).
BlackBerries are great with PIM, Contacts, Calendar, Tasks, and Memos do great with Outlook. at work, I administer a BlackBerry Enterprise Server. It is great for synchronizing everything wirelessly, without anyone needed to connect their BB to their computer at all. There is also an internet service. It works well for email. I believe you would have to connect via the BlackBerry desktop, which is similar to intellisync. it is a smooth interface and would be a smooth transition for you. Everything can be configured to synch up upon connection. It gives you an opportunity to decide what happens if there is a conflict between the device and your desktop.
For the 8703E, there is no option for upgrading the memory as you suggest. i can only add that the BlackBerry is very efficient in handling memory issues. It is less of a media capable device than a WM device or a treo.
It is great at what it does.
Good Luck,
Dave