I built a 4 Terabyte Raid (technically a striped array) with 8 of the new Hitachi 500GB Sata II drives and the Sonnet Tempo X Card.
It’s very quite and the Kona drive test shows I’m getting read / write speeds of around 475MB / sec. The Hitachi drives ship with a default of 1.5Gb/s per second which I left alone since the Sonnet card can’t handle any more. Once Sonnet comes out with their next generation card I’ll be able to turn the drives up to 3Gb/s per second and probably break the 500MB per second barrier.
I’m very pleased with the setup… but as everyone’s stated, building it your self isn’t for everyone. I backup every night on 3 Lacie 1.6TB drives to be safe.