Samsung Mantra M340 does seem to support sync BUT...
Virgin Mobile wont respond to questions on it as they would rather you BUY FROM THEM the ringtones , songs, pictures {via MMS charges} etc.
Samsung doesn't seem to acknowledge it directly on its web pages, instead pointing you back to Virgin Mobile
Third party vendors DO acknowledge that it works and even sell parts for it but wont support you if you run into trouble.
For example see:
USB CABLE+CD+CAR CHARGER FOR SAMSUNG M340 MANTRA - eBay (item 300340254554 end time Mar-17-10 16:06:47 PDT)
$16.35 Samsung Mantra-M340 Multimedia Manager (Samsung PC Studio + USB Cable)
USB COMPUTER SYNC DATA CABLE CORD FOR SAMSUNG ETERNITY - eBay (item 140389005731 end time Apr-05-10 22:44:31 PDT)
Part# of cables
Samsung USB Data Cable - (OEM) Original APCBS10UBEBSTD
You can give part number on EBay, Amazon and the guys above and you will get various prices from 55-$20 or so depending on configuration
At a minimum you need
Data Cable
CD of Drivers for PC
Samsung PC Studio 3 or higher
With this and a little experimentation you will be up and running!
Good luck!