See my signature for the link to Softmod Any Wii guide. It will recommend that you use a 2GB SD Card.
The 8gig card still might work, but the hackmii installer is picky about what it will run on. SDHC cards have a smaller chance of success than SD cards. Micro/Mini adapter cards have a smaller chance of success than full sized SD cards. Combine the two and you shrink the chances of success even more.