1. It's generally quite good, above 5mb/s for me. Use a simple drive with a single partition and you should be fine.
2. The .dat needs to go onto the drive you'll dump the files to.
3. Nintendo Wii ISO size: 4.37 GB (4,699,979,776 bytes) Nintendo Gamecube ISO size: 1.35 GB (1,459,978,240 bytes). I don't have the Dual Layer size off hand.