If I were building my SQL server again I'd go for:
C + Backup: 4*300GB SAS (RAID5)
Database: 2*300G SSD (Mirrored)
Log: 2*300G SSD (Mirrored)
this would give me 300GB for data and log, plus ~600GB for backups of my SQL data. it would fit in one chassis and I would not need an external disk array. the system disk for sqlserver does not need to be that fast -- once sqlserver is loaded into memory you should not need to swap (that will kill performance).
with three disks the best performance you could get would be to have a single system disk + mirror the other two for the database.
/RjL