1) is raid 1 slower than what? RAID 1 typically performs just like a single disk, even though you have 2 disks. RAID 5 should have better disk performance and give you n-1 disk performance with a penalty on writes and CPU utilization for parity calculations.
2) You sure can use driver-based RAID (aka fakeraid, softraid, onboard RAID, host-based RAID) for whatever the chip supports. There are some good reasons to do so and some good reasons not to. These have been covered many times in many threads already and right now I'm not up to rehashing that argument. Basically it boils down to onboard is cheaper and when it works it usually works pretty well, but it's not as good as a hardware controller when it comes to RAID 5 and is usually harder to deal with when it breaks.
3) RAID 5 and RAID 1 are both redundant and should be safe for critical data; However, RAID is not a backup. It is a way of maintaining uptime and preventing data loss in the event of one or more disk failures. It is susceptible to any number of other forms of disaster. Once more for emphasis, RAID is not a backup.
4) Possibly. You need to make sure whichever controller you use supports RAID level migration and then you'll be fine. Otherwise, you need a buffer location to store your data while you destroy and recreate the array.
I tend to really dislike most of the consumer NAS units when a cheap P3 will more than suffice for most file-serving needs. Maybe someone else can chime in on the Dlink you're looking at.
