always use RAID (along with regular backups of important data). disk drives fail, it's a fact of life.
I usually do something like:
10% in raid-1 across 3 disks - put OS & critical data here.
30-40% in raid-5 - put important data here (source code, digital photos, etc)
the rest no raid - not so important stuff here. downloaded movies, iso's, etc. stuff that you'd rather have space than safety for.
so if you bought 3 500GB drives, that works out to like:
50GB OS/critical data (plus 100GB wasted due to raid)
300-400GB important data (plus 150-200GB wasted due to raid)pl
750-900GB less important data
Also, an interesting note, temperature is important to drive life, but unlike CPU's, you don't want to get them as cold as possible. optimal running temperature is around 40-60C iirc, if you go outside that, either hotter or colder, drive life drastically shortens (this is based on a study by Google of thousands of disks)