RAID5, generally speaking, is a bad idea. Unless you have some very specific needs, RAID5 increases complexity, is more prone to hardware failure and administration errors, and does not automatically back itself up. If two RAID5 member drives fail at the same time, *all* your data is SOL. Individual drives, such as a bunch of externals, are probably a better choice for most people. And if two individual drives fail, you can still get data from the rest of them.
But people are nevertheless drawn to RAID5’s speed and size; it’s a great way to effectively have a freaking gigantic drive. It’s set and forget, easy to expand in the future (with some filesystems), and for reads is substantially faster than one drive. If one RAID5 drive fails, it’s an easy fix via drop-in replacement.
Some advice: when building a RAID array, use drives from different sources. Going different brands is probably a good idea, but caveat emptor: Company A’s 1TB drive will most likely not be the same ultimate size Company B’s. The available space will quite probably be skewed by a few KB or MB.
It should go without saying, but build your array only when you have all drives in hand. To more easily allow future expansion of your RAID, when it comes partition time shave a meg or so off your smallest drive. And when you decide to expand your array, buy a drive that’s the exact same model as one already in there. It will probably be cheaper than the newest model anyway.