Well, if I can believe Wikipedia, the GameCube had just 25 million sellers in 5 years. Wii should pass that easily by the summer.
Also, SMG is almost certain to pass SMS in the next few weeks, Twilight Princess passed Wind Waker long ago, Mario Party 8 could potentially outsell MP's 4-7 combined, Super Paper Mario has doubled Paper Mario: TYD, and even Mario Strikers has outsold it's predecessor. The only major Nintendo title not to outdo it's GCN counterpart is Metroid. This would seem to bode well for SSBB outstripping Melee's 6 million (and I still think it has a good shot at hitting 10).
It'll be a bit harder for the Wii to pass the N64 benchmarks (11m for SM64, 7.6m for Ocarina, 8.5m for MK64), but it should still pass it's total of 37 or so million sellers by the end of 2009.