Data transfer: What's helped me is using the OSX Documents folder correctly on the old computer. I replaced a old G4 tower running 10.4.11 with an Intel iMac running Snow Leopard; over the years with the G4 I had been careful to keep all data files in my Documents folder. I simply used our home network to move the old documents folder, relabeled "Docs from G4," to the new computer, and then moved what I needed. Nothing was lost, including years of MS Office files.
My wife had not been so diligent in keeping data files in Documents on her eMac (10.4.11). Some were loose on the Desktop, some in folders with their applications, and I even found a few in the Downloads folder. We recently bought her a nice iMac 21.5" and I spent a couple of hours gathering stuff on the eMac into a single folder that I could move over the home network. Again, success, but this time only after a little "pre-work" on the old computer.