Get the mmcmobile if you want to flash hacked kernel with high speed mmc modes, kingston 2gb seems to work perfectly. Sandisk 1GB is plain RS-MMC and therefore cannot be made to run faster. I would avoid it and go for Kingston mmcmobile one (1 or 2 GB). Dual vs single voltage doesn't mater. AFAIK linux kernel in N770 doesn't support lower voltage for dual voltage cards.