Depending on the load, I would add also separate disk/array for the tempdb.

RAID5 for backups is totally ok, but I think an important question is whether RAID5 is usable for the main DB storage? Perhaps maybe the same question can be asked for the log?!?

Another question that pops in my head is whether SSDs will change the game and if yes - how? Perhaps their speed will lead to simpler disk designs?