Eh well - I'm certainly no hardware expert - but Gamecube memory cards are flash based (it's basically an SD card), so you'll find that there is a correlation between the device's life span and how many write cycles it has had.
Of course since we are talking about something you've had for a decade - physical deterioration that will happen over the years is going to be a factor.
Being "8 blocks from full" in itself isn't too much of a concern. Some games are storage hogs and use all the blocks of a standard memory card from your first save.
Chao data cannot be backed up via normal means - You'll have to do something frowned upon by Nintendo, listed by degree of difficulty and/or risk:
- Cheat devices such as Action Replays usually have a save backup tool
- Wiis can be softmodded (homebrew etc) (thinking about doing this soon myself)
- If you're tech savvy - I doubt the card you modify in this way will be good to use though - I imagine you would have to mod a different card, then create a chao garden on that card and use the move function to move your chao over - then backup to your computer.
I've fallen out of the loop technology wise over the last few years so I wouldn't be surprised at all if I missed something easier
But all in all - Honestly I just wouldn't be too concerned about it.
I want to play so badly but I'm scared of my memory card dying. Please help 
