here it is the way to go.

Copy music files to a memory card
1. Insert a memory card.
2. In Menu mode, select Settings → Phone settings → PC connections → Mass storage → Save.
3. Press [ ] to return to Idle mode.
4. Using an optional PC data cable, connect the multifunction jack on your phone to a PC. When connected, a pop-up window will appear on the PC.
5. Open the folder to view files.
6. Copy files from the PC to the memory card.

Is it easy, just drag and drop, I prefer using Memory Card to store music files.. As to music file size and format, i did not notice any limit yet, well you may want to know more about how to Synchronise your phone with Windows Media Player here, direct transfer your Windows media LIB, music files to Your Phones or .

Using Samsung Jet New PC studio Software to Manage your Music Files

The user manual is made ready download and there have tips to guide how to transfer music files..and

Copy music files via Samsung PC Studio
1. In Menu mode, select Settings → Phone settings → PC connections → Samsung PC
studio or Mass storage → Save.
2. Press [ ] to return to Idle mode.
3. Using an optional PC data cable, connect the multifunction jack on your phone to a PC.
4. Run Samsung PC Studio and copy files from the PC to the phone. Refer to the Samsung PC Studio help for more information.

If you have not the PC studio for samsung Jet, download it here

and there are other 2 ways to transfer music files, via Memory card or Synchronise your phone with Windows Media Player