Are the problems that are occurring with the cards that the data is becoming corrupted? If this is the case does it sort the problem out with a reformat? If this is the only issue then i can just do backups of the data and reformat and restore the data.
If the issue is more serious and its completely destroying the cards then surely Nokia are legally liable to replace the card for you?
I have 2GB PNY cards in both slots that I bought for like $20 at Wal-Mart. So far, they haven't died, but I don't do a lot of intensive access to them, just the occasional load or save of a file.