For USB driver issues with Win7 x64, try installing the drivers found at this link:
Download Samsung Galaxy S USB Driver for Windows x64 Driver for Windows XP X64 / Windows Vista64 / Windows 7 x64 - Softpedia
Use the device manager to update the drivers (Go to device with yellow triangle, right-click, Properties, navigate to 'Have Disk' method where you can point it to the folder). This made Mass Storage mode work fine for me. (Kies is another issue).
Hope this helps!