Networks

oh, c'mon. let us know the os. and more particulars about the size and er shape of your network. it is so much more fun to guess then.
does this help?
since you don't i assume you on win2ksvr sp4 and small handful of 2k and xp workstations...
You only need to backup server1 if you can't afford to lose the data off it. when i have to do this, if my old system still boots, i try to replace the hd, that way got yet another backup and easy way to revert to old system.
why are you reinstalling? be carefull you don't backup baddies with the goodies. mark the backup media 'emergency only' 'possibly infected' or something, is my two cents...
do you want instructions for installing, securing or about promoting and demoting domain controllers (yes, you need to do this step unless you like me and have such a tiny domain/network you just recreate it)
here are the instructions for that:
How to promote and demote domain controllers in Windows 2000
http://support.microsoft.com/default.aspx?scid=kb;en-us;238369
instructions for everything else on there also...

sure backup the server . there is good info how to back up a 2003 server with the configuration/system files in mind . but if you want to have 0 risk , I would suggest an informal method := you do a bit copy ( image of the disk ) . Try the new image if its ok you can make the reinstallation the way you wish .

If you have 2 DC, and you have replication between the two, why would you need to back anything up? If you are talking about data, then of course you would need to back it up, that goes without saying! If you have your other server with all the FSMO roles, then blast server 1 away.