f you want to use Google Sync to synchronize your mobile phone with Google's address book and calendar, you will probably have problem with fact that upon synchronization, your mobile phone's content will be deleted (both contacts and calendar items).
I personally use my mobile phone (HP iPAQ 514) as most up-to-date storage of contacts so I had a big problem with that. However, I decided to give Google Sync a try. In whole process it cheated a little in order to preserve all contacts (and calendar items).
Please notice that while this whole procedure worked fine for me, you may not be that lucky. Perform backup!
First we need to copy pim.vol file from root directory of your mobile phone. This is place where most of your PIM data is stored. Before coping that file, you may want to turn off and on your mobile phone in order to flush all data in cache. This file is write-cached to enhance performance and to avoid wear-and-tear of underlying flash. Save that file to SD card - we will need it again later.
Synchronize with Google Sync. Instructions are at Google's site and I will not repeat it here.
Once synchronization is done, you will notice that your contacts are gone and replaced with those defined at Google's side. Now it is good time to turn off your phone, turn it on again, and copy pim.vol (one your stored in step 1) over pim.vol in root directory. If you get a message that file is in use, wait a little and try again. Once file is copied, turn off your phone and turn it on again. At this point, you should have all your contacts back.
If you synchronize now, all your contacts will get recognized as new and transferred to Google. From this point on, your phone is synchronized.