I'd make an image and deploy that, making users responsible for backing up their own files to a server somewhere. Although, with all the "windows geniuon" b.s. it's probably not as easy as it used to be to deploy images of an OS.
P.S. oh, and I'm also assuming that most of the computers are the same, i.e. same drivers will be required, but with that many computers, it's probably not the case.
namely I have to configure it to fix with different driver, right?
If you have different hardware, you pretty much have to do a complete install on each computer... there may be tools to help in this process but I don't know about them since I've never had the need to update that many machines.
More important part, is there any configuration i have to customize on my machines to meet windows 7 requirement?
Tips/tricky parts for transferring files back? for update outlook? etc.
This really depends on what applications people are using and where they're storing their files, you could write scripts to automate all the backups but problem is that people have a tendency of storing files all over the drive instead of just in their directories (major pain). With outlook you pretty much have the same problem, if people left their mail on the server, then it's not an issue, but if they downloaded the data (made data files), it could be anywhere in the system (i.e. wherever each user configured it to be).
also I was told that before updating, user has to backup their own data, is it the only way? automated updating without ruining files/data not gonna happen, right?
Like I said above, you could automate things, but you're bound to find that people stored files where you didn't expect and end up losing files for some. Maybe you can break up the upgrade into small groups of computers (let's say ten), run your backup scripts that should back up all the files that are in expected locations (user folders) then make images of their hard drives before doing the upgrade. Once you upgrade the systems, run scripts to put the files back. The users can then go look at their system to see if all of their files are there, give them a timeframe (say a week) to verify, if they're missing files you can go to the images of the old drive to look for the files. This will make the process slow but at least you won't piss off as many people.