The card has 13 contacts (two rows). Plain MMC has 7.

MMC4.1 i.e all MMCmobile cards currently on market.

MMC4.2 is newest and adds >=4GB support just like SDHC but I think you can't buy such cards yet. These won't work with current kernels but patch is trivial so once they are on market support can be added.

As for performace - unless you want to flash kernel which support high speed MMC modes it doesn't matter which card you have in N770.

If you decide to use such kernel than is matters if the card supports only 26Mhz mode (all mmcmobile/mmcplus cards must support this) or can run at 52Mhz too (this mode is optional). You can find this only by testing. There are mmcmobile cards that support only 26Mhz mode (i.e. only half of maximum speed possible in N770).

Also it depends if the card is stable with N770 or not. For me the Kingston 2GB works fine and can do the fastest mode. I am not sure about other cards. Definitely there is some reason why Nokia restricts MMC clock to 12Mhz in their N770 kernel so YMMV.