They are doing good.
If the attach ratio holds for 8 it could become a 10m seller one day. I'd say at this point beating Super Circuit is a certainty, beating Double Dash is highly likely, beating Super Mario Kart a possibilty, but beating 64 is a longshot. I think Mario Kart 8 can end up around the original and sell 8m, which is awesome if it happens.
Mario Kart 7 has also done wonderful, 11.5m is pretty big, especially on handhelds. It's not going to get to DS, but could get as high as 15m potentially. 13m is probably a safer bet though.
I hope Nintendo updates Wii's numbers soon (probably march like last year), it's still selling and could be coming up on 37m by now. That's insane for an exclusive.