I'm not sure if they already exist, but certainly they could. Jesus goes to prepare a place...or is it already there just waiting to come down? There are those saints at the throne asking "when?", so they are with God.But isn't the Eternal City called the "Heavenly City" and doesn't it "come down from Heaven?"
If so, could the streets of gold and the gates of pearl already exist in Heaven?
I believe we will be with Christ when we die, but I also think that the Heaven Scripture looks to is the Kingdom as it will be here. I lean towards D.A. Carson here (actually, Wright agrees with him on this part). Carson equated the city, the New Jerusalem, as symbolic of God's presence coming to dwell with man in this new creation (in one of his Gospel series...I think the last one). Heaven (our final dwelling place) is not up in the sky but on the new earth.
I should also note that N.T. Wright clarifies that he is speaking of our ultimate destination and not a state between death and Heaven. He leaves this a bit open in the book (that we die and whatever happens next our ultimate destination is Heaven).
I'll rely on your expertise here, though. It's enough for me to know I'll be with Jesus, and I really have not worked out much of an eschatology beyond that fact.