I'd consider a hardware RAID solution if at all possible.
Our primary backup machine is a dual-Athlon machine with a 3Ware IDE RAID card with 8 160-GB Maxtor disks (6 in RAID 5 and two hot spares in 3Ware hot swap cages). Works great.
Our primary servers are all Dell rackmounts with Adaptec hardware SCSI RAID; some are straight mirrors but most are RAID 5--mostly using 15k rpm Seagate drives (stay away from recent vintage IBM SCSI drives...I had three fail last year).
Some of our Oracle databases are quite large and the backup machine has no problem keeping up with the dumps from the 15k rpm SCSI based machines--the data trasfers are limited by the 100-Mbit/sec network (gotta look into gigabit Ethernet one of these days...).
I'm very impressed with the performance of the 3Ware cards and they're well supported in Linux and Freebsd.
I had an older machine at one point using software RAID 5 on 4 IDE disks on a single controller--not optimal but it ran fine for years. Performance degraded during substained data transfers (this was a stats processing machine) but then it only had a single 366-MHz Celeron CPU.
BTW, the trend in Enterprise computing is software RAID. The setiathome folks awhile back reported they switched to software RAID on their Solaris machines after a hardware RAID failure resulted in a total data loss situation. The hardware architecture of these sorts of machines are much better suited to software RAID than PC-architecture with a single IDE controller.
