Hot swap or online expansion is only available to host controllers, i think
Host controllers are noticeably faster, the processors on them are highly specialized units. They manage the command queues, and buffer the I/O properly so the main system is relieved from these tasks.
What you must assess is your needs in terms of load: what are you using your storage for.
If the intended use is database server, file / app server with many concurrent users, then SATA is not enough - SCSI RAID is the only way at the moment (until SAS is out). Multiuser loads are not comparable to single user ones.
If it's only for a single user, but you need lightning speed, take a Raptor, and backup it (like, nightly) on a USB/Firewire/NAS box (with a good old slow but huge unit inside - take a Seagate for reliability). The idea is only to run your backup solution when doing the actual backup, like nightly for example. It will be a LOT cheaper than a full-time redundant array, and even more performant (remember RAID 5 comes with a performance hit, for a single user).
If you're craving for speed stripe 2 Raptors... but really a single one is already blazingly fast. Or if you're paranoid mirror them. Either way dont go RAID5 for a single user.
RAID5 is meant for high load, multi user scenarios, where the time-to-deliver becomes essential when 400 users are hitting your server at the same time, or 10 marketing managers hit your database with huge and malformed SQL queries.
Define your needs, then we can define a more precise solution
