RAID1, mirroring, nearly doubles the throughput when reading, but slightly decreases the write performance. Since most of the time you write a file once, but read it many times (think installing a program vs. launching that same program), it results in a net improvement. But the real reason for RAID is to prevent data loss in event of hardware failure. Either of the drives in the mirror set can die, and I'd lose nothing.
My system has two 136 GB, 15k RPM drives in RAID1 which hold the OS and programs. Then two 1 TB, 10k RPM drives, also in RAID1 where I store data. I'm waiting for the 2 TB SAS drives to become available as my 1 TB array only has 9 GB left at this point. I have eight drive bays, and an eight channel SAS controller (with hardware mirroring) so I still have room for expansion.
