Prepare to update a lot. If you still have the original hdd it might take a little longer than expected too. GT5 has a lot and has a mandatory install. GT6 has updates then has to... apply updates when you still load it? I'm posting this while I'm still waiting for GT6 so there might also be an install or more updates.
OH yeah, and if you're planning on modding GT5 learn how to update to a specific version or something. I haven't done any of that