I'm using Matrix RAID with four 1TB Ultrastar drives in a 558GB RAID 0 array and a 1721GB RAID 10 array, and consider it a great solution for both performance and redundancy.
Four GP drives on a Matrix RAID controller, I imagine, would run 250-300MB/sec sustained transfer speeds with a ~9 second access time, on the RAID 0 array. And in RAID 10, I would guess, should hit around 125MB/sec sustained and ~12ms access time. Both arrays would beat a Raptor or Velociraptor by a wide margin in throughput and lose by a few milliseconds in access time. Noise would be likely lower than with a Raptor and higher than a Velociraptor. This is all speculation, but fairly educated speculation.
Hope it helps!