2. Yes but you can do so while you are installing. If you want to preserve your home partition (if it is on a partition of its own, that is), select it during install but choose NOT to format it.
3. Depends on the type of application. Multimedia, databases, number crunching etc. would be among the first to benefit from 64 bit, other applications won't be any faster at all.
4. Yes. Place your media on the hard drive and get an iso editing tool (like acetoneiso) to extract the vmlinuz and initrd files. Place those in the boot directory of another installed distro (even XP should work) and put a reference to them in your GRUB that looks like a regular GRUB entry. Reboot and select the new entry.