OK, I gotta get to bed, but I'll post this for you to chew on and for other people to correct me on.
OK, here's the quick rundown on RAID from someone who's never dealt with it personally and doesn't know everything there is to know about it.
RAID is NOT a backup solution. If you delete a file from a RAID volume, it's gone on all of the disks involved in that volume. There's no protection against accidentally deleting or modifying a file at all (which I think would be a good thing considering what you're using these drives for). If you're thinking RAID is used as a backup solution, stop right here.
RAID 0 aside, it's effectively a way to prevent data loss from a hardware failure (i.e. one disk dies). You mention wanting RAID 5 with 2 drives, but RAID 5 requires at least 3 drives. The best you could do with 2 drives is RAID 1, which wouldn't give you any performance benefit at all as the two drives just mirror each other. For the best read/write speed benefit while maintaining a 1-disk fault tolerance, the best you could do it pack as many drives as you can into the enclosure and set up a RAID 5 array.
This brings up another issue though: How fast will the actual maximum read/write speeds be? This is important because even if your RAID array can somehow read at 200 MB/s, you have an interface that can only handle 800Mb/s (100MB/s) on a good day. This is assuming a best-case scenario in terms of speed and assuming the chips in the enclosures are VERY good. Basically, you can throw as much money as you want at the problem and cram as many disks in there as you can, but at some point it's going to make no difference at all. Where is that point? I have no clue.
If it were up to me, I'd set up the enclosure with a fault tolerant RAID array (RAID 1 or 5, depending on how fast I wanted it to be and how much space I needed in the volume) for everyday use. That way if one drive takes a shit one day, you can just replace the dead drive with an identical one, rebuild your volume, and be on your merry way (hopefully).
To account for actually backing up the data on that array, I'd also have another completely separate drive (a single 2TB/3TB drive or another array if you wanted to be super paranoid about disk failures or if you needed more space) set to do scheduled incremental backups at whatever interval you want. Speed shouldn't be your primary concern here as you shouldn't even think of working using your backup drives (unless you're constantly changing several GB worth of data between backups and the actual duration of the backup process becomes an issue), so worry more about fault tolerance than speed benefits. Depending on how paranoid you are, this can get awfully expensive. It all boils down do exactly how important that data is and how much you really want to invest to protect it.
Of course, I have no clue if this is for professional purposes or not. I trust you can figure out for yourself exactly how valuable your data is and to what (possibly insane) lengths you'll go to protect it.
EDIT: Take a look at the Wikipedia article for a bit of light reading on the subject if you want. http://en.wikipedia.org/wiki/RAID
Actually, glancing over the article again, it would appear that Mac OSX doesn't support software RAID 5 so (Please, anyone correct me if I'm wrong!!) I think you'll need to ensure that the enclosure you get supports it at the hardware level. I guess you could try a RAID 1+0 array which would require 4 drives for any benefit if you have to use software RAID.