1. Yes, 64-bit is the preferred version
2. Yes, it will completely format and overwrite any info on the drive.
3. W7 Premium $119, W7 Pro $199, W7 Ultimate $219.
4. Yes, the release candidate is good until June 1, 2010
5. Don't know if it's faster than XP, but it does perform better than Vista.
6. I really can't answer that.
Most everything you need to know about it is available from here: Windows 7 Release Candidate