i'd say 80 gigs total capacity for raid 6+6 using 16 x 20gb drives
tolerance... thats nasty to explain but i'll try.

Top Level:
4 - 80 gig raid 6 (which drops to 40 due to parity) - can lose two of these as a whole

any One of the 80gb raid 6 you can lose two of the individual drives in the raid.

so you could lose up to 8 drives (2 per individual raid 6) and still be able to restore your data on each of the 4 raid 6 pieces. BUT you're also allowed to lose two of the individual raid 6's so you could tack another 4 drives onto the total taking out two of your raid 6's. Grand Total by my strange calculations = 12 down'd drives

but you'd have to be doing something crazy to nuke 12 drives in a go!

did i get it right? if so, gimme another one!

(note: i'm at work and extremely bored.)