Before you decide to buy a new card. I also thought a while back my memory card was busted, but I decided to play around (quite a few posts back).
I connected the phone to my laptop and used standard Windows Checkdisk (right clicking on memory card and choosing properties>tools> error checking). I scanned for file system errors and bad sectors.
It took quite a while, but removed phone and tested. All my songs and data and apps that didn't work for that short period was fine afterwords. Haven't had any issues since.
This was Windows 7