I know that Gamecube games use 4-59 blocks for their saves ...
So for instance, if a game uses 59 blocks and the Memory Card has 59 blocks, the first save made by the game fills the 59 blocks of the memory card, what would happen when there is a second, third, fourth ... save ? Does the 59 blocks of the memory card gets overwritten every time there are newer saves ?
I bet you would not have asked the same question for another data storage medium. You can only put files on there if there is sufficient space left – else the computer will throw an error. If one file takes up the complete medium, you have to delete it before writing anything else.
If the game which fills the MC on its own wants to update the save data, it will most likely not overwrite the complete card each time but only write the portion that has changed (saves time and write cycles).
This can be actually seen on the first DS Pokemon games. When saving the game for the first time (or after changing data inside the Pokemon boxes), the game has to write more and even shows the message
"saving a lot of data, don't turn off the power"). When simply saving while not much has changed saving is quicker and doesn't show the message.
Nothing of this should be any different on GC compared to other consoles or computers.
Don't get confused by Nintendo's strange measuring units. For whatever reason they decided to not write normal memory units on their products in the past but used some fantasy-blocks.
The Memory Card 59 has 512KB. Each block is 8KB in size – 5 are reserved for whatever (similar on PlayStation 1: 128KB Memory Card divided into 8KB blocks but only 15 are accessible).
Memory Card 251 has 2MB, Memory Card 1019 has 8MB (and unofficial MC can have up to 16MB with 2043 blocks usable).
To make this block nonsense even worse: "Block" is not even a clearly or well-defined unit. On DSi, 3DS and Wii a block is 128KB in size compared to the 8KB in the old memory cards.
Good luck and have fun!
