I'm new to the droid scene so I'm not too up on the lingo, but I had a similar problem with my Acclaim where the SD card wouldn't show up on my PC. My OS is Windows 7 64 bit. My carrier is USCellular.
After running through all the help offered here to others and nothing working, I finally turned my phone off and on again while connected USB. As the phone was booting up, my PC picked up on the drivers and all is well now.
I'm at a loss as to USB debugging. Some say turn it off, some say turn it on. For me, I have to have it enabled. I hope this helps.