I have had a look in the User Manual to determine what sort memory card is fitted to the 276C, but the Manual only refers to it as a 'Data Card', do you know what sort of memory card you have?, current cards include MicroSD, (also called TransFlash), SecureDigital and CompactFlash, but in the past there have also been XD Card, SmartMedia and a few others, it is also possible the Garmin came up with their own type of memory card. If your card doesn't have any of the above names on it would it be possible to photograph it?, or if not measure the card and post the dimensions on the thread, so that I can find out what sort of memory card it is
EDIT
It looks like Garmin made their own 'Data Card' for this unit, some of them were 512MB, here is on on Ebay, note that there is a warning that 'Early Units' in the 100 and 200 series wont work with this Data Card :-
http://www.ebay.com/itm/GARMIN-Datacard-512Mb-Data-Card-512-Mb-GPSMAP-Latest-Release-High-Speed-Memory-/280945021390
Note from Ebay Post :-
IMPORTANT!!! Some of the older 100, 200 series may not be compatible with 512Mb datacards but only with 128Mb or smaller. Please contact garmin tech support to confirm compatibility of your unit with 512 Mb datacard.
However, they do say that 512MB will work with the 276C
BTW
To answer your question, an SD Card is not going to be compatible with this data card