I wrote SdFat, Fat16, and the base code for SD.h. These three libraries have similar code but the new beta of SdFat supports more SD cards than other libraries so download and install SdFatBeta20120825.zip http://code.google.com/p/beta-lib/downloads/list.
Run these three examples: QuickStart, SdInfo, and bench.
Post the error codes if any of the above examples fail. Also post the results of the bench example if it runs without errors.
If all examples run O.K. change this line in bench.ino from this
#define FILE_SIZE_MB 5
to
#define FILE_SIZE_MB 25
And rerun it. This should do a write stress test.