You might want to rethink that. At the very least get the new phone before you sell your iPhone. You can put your iPhone on T-Mobile for about $80/month with unlimited everything. I picked up a Droid and am struggling to have the patience to get through the 30-day trial period. Coming off an iPhone, any Android phone will be a tough pill to swallow unless you really enjoy feeling like a beta tester. At least on the Droid, everything feels half-baked and half-complete. The iPhone may be a walled garden, but at least it works -- flawlessly.
That said, and since this is an overland travel forum, there are some really cool GPS apps for Android, both free and paid.
"Follow your bliss" ~ Joseph Campbell, mythologist