Yes, thats how you recognize it, card has two rows of pins (13 in total, and in fact 8 data pins, not 4). Plain (RS-)MMC has 7 pins in total.

But those extra data pins are not used at all in N770 (at least in the one I have). What matters in N770 is maximum frequency card can do which is 20Mhz for rs-mmc but 26 or 52MHz for mmcmobile. Due to omap chip limitation this translates to 16 vs 24 vs 48 MHz used. You only see the speedup with hacked kernel, though.

The kingston 2gb mmcmobile was a good choice for me. It can run at 48Mhz just fine in my device. Some people reported mmcmobile cards that support only 26Mhz mode, not 52. When I bought it even the price was OK (i.e cheapest from all rsmmc/mmcmobile). Currently it is for approx. 30EUR here including VAT, not sure how cheap other 2gb rs-mmc/mmcmobile cards are.