I am planning on a new computer for Photoshop (i7-950, 12 GB RAM, Asus Rampage III Formula Mobo) but I am uncertain which of Tom's benchmark charts to emphasize. My initial thoughts were to get a 150 GB Velociraptor for the OS and program files. Two 500 GB Western Digital Black drives in RAID O with a 64 GB partition for the Photoshop Scratch file (remainder for internal backup). And finally a fourth drive, 1 TB data storage (photo files, music, etc.)
After looking at all the hard drive benchmarks I am totally confused which drives to purchase for a set-up like this ( I am not too interested in SSD because of cost)
I find it a bit incongruous to say that SSDs are out of your price range when you're considering spending money on a Velociraptor and 12GB of RAM...
What kind of Photoshop user are you? Do you process large RAW images from digital cameras, and do so using batch actions? Or do you generally process up to a few individual files at a time?
I use Photoshop CS4, mostly for individual JPG files, and my system only rarely gets close to 4GB of memory use. I actually have 12GB of RAM for other work that I do, but for Photoshop, at least the way I use it, that much memory is definitely overkill.
My experience is that the speed of the work drive is pretty much irrelevant for light Photoshop use. I've run some tests with the work drive variously located on my SSD and on my bulk storage drive - a "Green" drive that has relatively low performance. There is no noticeable difference in the interactive speed of Photoshop. I can open multiple files just as fast with the scratch files on the slow bulk storage drive as I can with them on the much faster SSD. Changes are applied to the image as quickly as I can invoke them using the mouse. While monitoring the disks I've observed that the scratch drive activity actually occurs after the files are loaded or modifications are made to them - so Photoshop seems to be very smart in scheduling its workload.
On the other hand, the system does boot up much faster and it loads up Photoshop and all its plugins much faster when everything is installed on the SSD as opposed to being installed on a hard drive.
Most of my Photoshop images are 350 MB scans of 6x7 transparancies and B/W TMAX 100 film. Final file sizes (all layers) are often 1.2 to 1.5 GB. Sometimes I stitch 3 of these 350 MB files into a pano. I often wait well over a minute to either open or close these on my present (ancient !) 5 year old two drive Dell Computer (4 GB RAM with 3 GB boot.ini switch). Sharpening can take several minutes alone. Lately I've been stitching 21 Mpix RAW files (Canon 5D, rented) ... 3 to 8 files depending on use (focus bracketing, panoramas, etc).
... so, to answer your question, I do stress Photoshop; definitely not 'light Photoshop use' and most 'changes' are not as mouse speed !
OK, it helps a lot to understand the requirements before giving advice. Unfortunately what you're doing is outside my Photoshop experience, so pay more attention to any other suggestions you receive from someone who's "been there".
The important thing to remember about RAID 0 is that it increases the transfer rate but doesn't improve the access time - so it's a good choice to use when reading a few large files (as opposed to a lot of small files). Since your files are so large, transfer rates for them are going to be important and you may want to use a RAID 0 set for the files themselves as well as for the scratch disk. Of course you'll want to have a good backup strategy for them. (Don't think you need the backup strategy just because they're on RAID 0 - ordinary drives fail too and so of course you need a backup strategy either way.)
For the scratch files, I don't really know how Photoshop uses them and therefore if it's more important to have a high transfer rate or a fast access time. If you use a Velociraptor for the OS disk then don't rule out testing it out for hosting the scratch drive. As long as you're not running other programs that are using it at the same time you may benefit more from it's faster access time than you would from the faster transfer rate of a RAID-0 volume. It's also possible that you'd get the biggest benefit from using all the drives you have for scratch space. Unless there's someone out there who knows how Photoshop uses it's scratch area I'd do some testing with various scenarios to see what works best for your workload.