i'm not sure if this would help. After I'd put on the new mmc and try to format it error "mmc corrupted". Luckily I had my Nokia 3650,60 handy and pop it in there to format it. It works fine after that.
Seems to be one of the many many errors/bugs in the 6260. Happened to me too - bought a NCP branded RS-MMC with 256 MB, tried to format in the 6260 - phone hung. Still could turn it off tough, after that it said "Memory card corrupted".
I put it in the reader on the PC and formatted it there (FAT format), worked flawless ever since (anyway copied everything from the old card before using, so the usual folders were present and no errors).