Two ways to do it, and both are easy.
1. Use HotSyncing:
Perform a HotSync (make sure the contacts/address conduit is enabled), do your hard reset, and re-HotSync (make sure the contacts/address conduit is set to "computer overwrites handheld").
2. Use your SD card:
a. If you don't already have one, get a backup application. This is something that is universally strongly recommended. I use
BackupBuddy and love it. Do a backup, do your hard reset, run the backup app from the card (if using BBVFS, make sure to select the Advanced menu option), select everything starting with the word "Address," tap Restore, and tap Yes on the confirm window. Done. Then backup again to save the new state of your Treo.
b. This can also be done using the excellent (and free)
Filez with a little more know-how. Launch Filez, tap the View and Edit Files button, tap the triangle next to Internal, select everything starting with the word "address (a checkmark will appear), in the menus select Item > Copy..., tap the name of your SD card (the triangle will turn black), and tap OK. Confirm the stuff copied by tapping the triangle next to your SD card. You should see the "Address" files right there at the top level directory. Then do your hard reset and reinstall Filez. Then do the same thing, but copy the stuff from the SD card to internal RAM. Tap Yes in the confirm window. Done. You can delete the stuff off the SD card when you're sure everything copied properly.
I still advise using a backup util, though.
Cheers.
-K
P.S. I suggest option 2a or 2b. You have more control and it's more predictably successful.
P.P.S. This will also work with calendar items, todo's, memos, pretty much anything that has a .pdb file that resides in RAM.