That's just what I was interested in hearing! Two questions: does the Velociraptor make much noise, and why do you not use RAID for your main storage? I may have misunderstood matters, but since it's the time taken by PS to save large files which is the main prompt for me to upgrade, I wonder how much I'd gain if I used a setup like yours.
Jeremy
The Velociraptor makes some disk seek noise when the heads move, but is not terrible. I've actually used the Hitachi software to disable Auto Acoustic Management (AAM) to speed up the drive even further. It makes them a little more noisy, but that doesn't bother me.
I don't use RAID 0 for my main storage since my backup schedule is not rigorous enough to allow for the extra risk. I use the 1TB drive as primary, keep duplicates on the RAID 0 array, then off site backup every few weeks or so. RAID 0 is most important for the speed of scratch disk, where Photoshop will constantly read/write to the disk when it runs out of RAM.
I don't notice any significant delays when saving large files (1GB+) from CS4 to the 1TB drive. I think your delays may come more from the fact that you are running a single drive setup and that many resources (OS, PS, and saving) are all competing for use of the drive at the same time. SATA transfers from drive to drive are very fast, but things slow down when the drive has to both read and write from the same hard disk.
In your shoes, I would look at buying two WD Caviar Black 640GB drives and putting them in a RAID 0 array. Partition off the first 32GB stripe of each drive to dedicate exclusively for photoshop scratch (64GB total). The remainder of the drive can be for storage since you back up regularly. Keep your existing hard drive for the OS and programs, or if you want to speed up the overall performance of the computer and get faster program launch times, swap out your primary disk for an SSD to use in conjunction with the RAID 0 array.
The primary thing you need to do though is move to a multi-drive setup where one drive can be dedicated to OS/Programs and another drive or RAID array can be dedicated to scratch disk/storage.