1. RAID6 is more of a factory in large arrays when you need to account for the possibility of loosing more than one drive. Raid 0/1 will likely be the best performance assuming the extra storage space provided by RAID5 isn't needed.

2. I've got a 3ware 9650se raid array with OLDE seagates (over 4 years old) that manages to sustain 150MB/sec in Raid5. Pretty sure you won't have trouble streaming HD to and from that. It was more of a concern with the old IDE drives that could barely manage 30MB/sec (and got even slower in RAID). Newer 4 drive Raid5 Arrays should do even better so the array controller isn't a issue for a single user.

The Hotspare is very nice indeed, i find that its a function included in alot of todays controllers

For the drives you listed you really don't need something like the 1680xi, they aren't fast enough to really stress it even with 8 of them.

3. This drive is a desktop drive and rated as such, not for 24x7 useage (keep in mind), if this is a storage array with 24x7 uptime you will want to look for "enterprise drives" which also typically are validated on RAID controllers.

4. If you choose a external enclosure rather than getting a case which can simply accomodate the drives with hotswap bays you are pretty much forced to get controllers with external ports.

How many drives, what array size and what useage are you shooting for (or the MB/sec you want)?