First off, that's too many variables for my taste. I would upgrade to 9i on Sun first, and then worry about swapping OS/Hardware.

If your downtime is really that minimal, I would look at multi-master replication. I would setup both boxes where users could do updates on either box. Then I would move a trial group of users to the new box to verify functionality knowing that I can switch them back to the old box should something go wrong. I would then switch everybody over to the new box and keep the updates going to the old box so I have a fall-back plan.