1. Is it okay to partition an SSD? Let say I bought a 256GB SSD and I want to divide it into several partitions. One for the running the OS, another for storing system image backup and other files (e.g. videos and mp3).
Yes fine to partition, however you don't need the "SSD speed" for the image backup and videos and mp3s. You will gain nothing apart from silence, and a large hole in your wallet.
2. Does SSD require a power cable connected to the PSU?
Yes, standard SATA power connection.
3. Does an SSD using NTFS file system require defragmentation once in a while?
Yes in theory, however in practice, de fragmentation of a SSD reduces its lifespan, so should not be run.
4. How long is the lifespan of SSD? Is it longer than HDD?
5. I heard that the read-write speed of SSD will become slower after a few years. After that what happens? Can I make the SSD faster again by reformatting it or get a new one?
They do get slightly slower but with an O/S, controller (no RAID) and SSD that support TRIM, they run at pretty much the same speed all their life. If you don't have access to TRIM, you can zero the entire drive, and that will restore its speed.
6. Can a SSD crash like a HDD would? If it can, then I still have retain an external HDD for backup purpose.
Yes, it is electronic. Nothing stops firmware problems, power cuts, lightning etc etc.
7. My mobo only supports SATA2 (3Gb/s). Is it good enough for SSD?
Depends on the SSD, some can use more bandwidth than the SATA2 specification copes with, however, a saturated SATA interface would give a might fast transfer anyway.
8. Any signs of manufacturers are starting to produce 3.5" SSD specifically for the desktop?
OCZ did one, but I don't think they will catch on as a normal product, just because they have all generally been 2.5" for so long now.