I'm usually teetering around 100 blocks free. Whenever I start to get close to empty I just take the least played games and slap them on a 2gb SD card.
I don't ever care to own an N64 again, but do realize the kiddo's like zelda and Mario games, so I downloaded Mario64, Zelda: OoT, and Mario Kart64.
Also got Ys Book I&II, Gate of Thunder, and a few others taking up space... lol
Just got Mega Man 9, pretty pleasantly surprised about that one. Pretty imaginative levels and game design.