Miaghstir: I have yet to get a RAID or similar (looking at a Drobo, but they're still too expensive for me). I rely on single HDDs, single copies of the files, and recovery programs when drives fail. Maybe I should move to RAID 0 to be even more adventurous?
Arkose: RAID 0 is significantly faster than even the best individual drives (although not as fast as solid state drives, of course). If one of the drives fails you lose all the data--and because of how data is arbitrarily split between the drives it's not easily recovered--but you're in a similar situation when running from a single drive that fails. No matter what drive arrangement you are using you should have a separate backup drive for any really crucial data.
You can freely mix and match different drive sizes for RAID, but the resulting drive will be a multiple of the smaller drive, e.g. if you use one 250GB and one 500GB, striping them will only create a 500GB RAID 0 (250GB x 2), whereas striping two 500GB drives would give an impressive 1TB of storage (500GB x 2).
IDE drives can be run in a RAID with a bit of work, but I wouldn't even consider it; even for drives with the same RPM rating the SATA version is actually faster, and this benefit will carry over to a RAID environment too.
EDIT: of course the most important factor to consider with RAID is whether you can actually justify the investment. If you have a couple of big hard drives already it's a sensible way to use them, but if you would have to purchase an additional drive to have something to make a RAID out of it might end up being better to pay a little bit more to get a much larger drive to mount alongside your existing one (especially considering the size limitation of a RAID to the smaller of the drives used).
Yeah, I know, that's why I said "to be even more adventurous", double the chances of complete failure compared to now.
A Drobo however, is like RAID 5, only you can use the full size of your drives (except the largest one). Any disk fails, you can simply replace it with another as large or larger (and if larger, you get to use the extra space as well).
I don't have any really crucial data. Sure I'll be sad if I lose stuff (which is why I'd like it to be slightly more safe, yet still easily accessed), but it's really no big deal, it's just entertainment.