You can upgrade member servers first without any issues. However when you are preparing to upgrade AD itself you will need to perform the following pre-upgrade steps in this order first before actually adding or upgrading a domain controller:
1 – AD must be running in Windows 2000 or 2003 native mode and not in any mixed mode (2008 does not support mixed modes)
2 – On the server that holds the schema master FSMO role run adprep /forestprep
Once you do that make sure you allow enough time for the update to replicate throughout all of your existing DCs
3 – Next, on the server that holds the infrastructure master FSMO role run adprep /domainprep
The adprep utility can be found on the Server 2008 DVD or downloaded from Microsoft.
Also, if you do not know which server holds which FSMO roles you can find out by logging onto a current domain controller and running “netdom query fsmo” from a command prompt
Good luck with the upgrade