So with RAID 5, any two drives go down in an array and you're fucked. RAID 6 bumps the total amount of parity to two drives total, so it would take three drives to go down to hose your setup. They don't have it set up so that you can choose how much parity you want, though? I mean, RAID 6 sounds good enough for me in terms of redundancy, but there are some people who might want even more.
RAID 4, since the parity is all on one disk, 5 drives could fail, and you would still be golden as long as one of those isn't the parity disk. On the other hand, if the P. drive and any other drive go down it's game over. I'm sure you could specify two drives for parity if you wanted to, though.
In terms of speed with 3 drives, you would get three levels of striping with 5/6 so they would be faster than 4 where you would have two levels.
For capacity, if they're 120 gig drives, you'd have 240 with 4 and 5, 120 with 6.
I'm rethinking staying with 3 drives. Getting 4 of lower capacity would crank out some sweeeeeet speed on 5/6, or give you as much protection as a plate mail condom on 4. The paranoid/obsessive compulsive madman part of me says "4 disk raid 4 with double parity; ain't nooooo doubt about it!" but it's not like the stuff on my drives is particularly volatile, I'm just lazy and don't want to burn periodic DVDs, or spend time doing recovery. I would like to have double disk fuck up protection though. Most of the time it's not even simultaneous, you'll be using your system while waiting for a replacement to come and another drive decides to shit its pants. I think RAID 6 sounds like a winner, especially if I can score 4 160g drives for a good price.
e; actually now that I think about it, RAID 4 isn't really all that good or more protective compared to RAID 6, at least at the # of disks and amount of redundancy I'm aiming for. Oh well, it was only the retarded part of me thinking anyway.