I just upgraded my computer hardware, so I'm typing this on a shiny new computer with 4 cpu's, a terrabyte of hard drive, and 8 Gigs of memory.
Getting all the stuff from the old computer to this one is still harder than it should be, but is easier when you upgrade while the old system is still working.
What ought to be true is that if you move the /home directory, install the same set of packages, and import the data from the database, you should have a working system.
Some people claim that you can just copy /etc and /var to the new computer and then the new system will work the same way the old one did. I didn't find this to be true, and I've been hand moving the things from /var and /etc that I turn out to need, or reconfiguring the new system. Part of why this is less true for me is that the new system is a 64 bid install, and the old one was still 32 bits.
In any case, when you have the luxury of the old system still working is a good time to check that your backup procedure is working, and to add things to it as you find yourself manually moving something from the old system to the new one.
The most embarrassing hole in my procedure was that I found my system for entering lilypond into emacs via USB keyboard didn't work because I'd installed a little program it needed in /usr/bin (which should be only executables from the package management system, and doesn't get backed up) instead of /usr/local/bin.
I don't yet have either gallery2 or wordpress working right on the new system, but the old system seems to have the same problems, so it probably isn't the backup procedure.
My own backup procedure is largely rsnapshot, along with some scripts that back up databases and the websites that are hosted elsewhere. This gets everything you need (as long as you tell it the right files to back up), but is fairly large and cumbersome, so one of the things I'm missing is recent off-site backups. It backs up to a 1 terrabyte firewire drive. Each backup takes up about 160 Gigabytes, but the files that are the same are hard linked, so 10 backups are only about 200 Gigabytes.
Anyway, I'm very happy with the new system, because now when the backup procedure starts I just barely notice, instead of having to stop what I was doing on the computer and go get a cup of coffee.