I strongly recommend that you go to the post at the top of this forum page titled "Where are all the stickies? In here." Here is the link:
http://forum.ppcgeeks.com/showthread.php?t=32382
The first post in the thread is the upgrading guide by GAmedic. It is complete and straightforward. It explains the how and why of upgrading your PPC.
The primary reason to replace the radio on the earlier versions of the 6800 is to get the GPS working. The most recent ones, at least from Verizon, have a newer radio ROM and can be tweaked to get the GPS working without replacing the radio.
Good luck!
WorkerB