I've downloaded so many virtual console games that I'm about to run out of space on my system. The SD card I have in my Wii is a kingston, and I've got another that's a generic microSD with adapter (CUBE memory, inc). I've offloaded and reloaded games back and forth from each without any issue.
To put it into perspective, I've got about 5 channel slots left on my Wii before I run out of them. You really have to download quite a lot of stuff before the system memory runs out.
I sure wish the SD card acted like a system memory expansion, because shortly I will be forced to pick and choose what is loaded (playable) on my system... for the time being, I'm just avoiding buying any more virtual console games.
Secret of Mana came out for $10 a few weeks ago, though, and I couldn't help myself there. That's just a damn good game for $10.