Can you confirm:
- your repserver is being moved from 2003 to 2008
- your ASEs/RSSD is staying on 2003 (ie, you're not going to 2008 for ASEs/RSSD)
Assuming the ASEs/RSSD are staying on 2003 ... are these in-place upgrades or are you planning a dump-n-load from an 'old' 2003 machine to a 'new' 2003 machine?
How much downtime can you afford for your ASE instances?