one thing i learned from doing a very LARGE db, was to trial run the import first.
there are pitfalls that come up with very HUGE post tables for example. things like mysql running out of temporary storage space or the like.
now your hosting environment might be such as to not have issues, but mine did. so i just did a DB backup.. zipped it up.. and loaded it locally on my windows box (running wamp).. and tried the import/upgrade.
it was good to do that, just to get a feel for what might befall me on things like an altertable on a POSTS table that approached 1gb in size..
So you might at least give that a try to see how it comes out, then prepare your members for the REAL deal and let it rip..
oh and i actually found that it was faster for me to do the following:
1. close the real board
2. do a backup (using cmd line) to a .gz file
3. grab it to my local system
4. do the upgrade
5. backup the file locally (.gz)
6. send to live board
7. unzip and import fresh
8. open board
reason being, my local computer is a beast in terms of CPU, Ram and the like (QX6850, Raid 5 Raptors @ 10K RPM, bla bla).. it churned thru the process in nothing flat.
in the end, the process took less time than running the upgrade script on the real host on a closed board.
your mileage may vary..
Good luck to you,
SD
