That is not possible to "overload" you memory card, perhaps it's time you research the difference between byte and bit instead. Quick pointer for you, 8 bits = 1 byte. Your sd card is in bytes (gigabytes) and nds roms are labeled in bits (ex. 256 mbits). Do the math and do a search next time for a question asked over 2000 times on this forum.
8 bits = 1 byte, that's correct (8 b = 1 B)
what changes also in the way of counting memory is the meaning of prefixes like "k" , "M", "G" (note that k is lowercase while M and G are capitalized.
for ROMs these prefixes are based on raising 2 two some power (I'm not English, I hope you understand); i.e. 1k =2^10= 1024, 1M = 1024*1024 = 2^20 = 1048576....
For SD and CF makers 1 MByte = 1,000,000 bytes, 1GByte = 1,000,000,000 and so on;
a 512Mbits ROM is 64 MB (MegaBytes) as said by azndragonguy115 , but with M based on powers of 2;
so you will be able to fit only 7 (and not 8 !) 512Mbits games on a 512 MBytes SD or CF memory card