i believe dumbledore said in book 5 that kreacher didn't know everything and they kept important stuff from him or something to that extend. but he'll still know some stuff.
also, i agree, grimmauld place is not just gonna become unimportant. it played too big of a role in the last book and they spent too much time on it. maybe harry could live there after school since he won't stay with the dursley's and he doesn't have any place to go (sure, the weasley's, but he'll want his own place and yes, he has plenty of money to get his own place) on the one hand i'ld say he'ld want to live there with it having been sirius' who he loved very much, on the other hand his (sirius) whole family was rotten and there are too many memories in that house.
i do remember that bellatrix and narzissa are sisters so who kreacher would go to i don't know ( malfoy's i guess). and i believe someone said it was the malfoy's kreacher ran off to.
sorry that was kinda in a hurry.
bianca