Have you determined that this problem is not caused by your graphics card? Most of the time this problem can be the transfer of data from input devices through the processing unit to the graphics GPU and graphics RAM.
Im curious what 15000rpm drive are you looking at? Ive rarely seen these drives unless it is SCSII The heat from a drive spinning at 15000rpm would be like running a mini BBQ!? A Solid State Hard drive is ideal however with your spec you shouldnt really experiencing this problem directly from your hard drive.
Have you tried to increase settings within photoshop itself like how much RAM it is allowed to use/And configuration for your graphics card (Enabling hardware acceleration, 3d now, and other features within photoshop). I honestly do not think that a new and faster hard drive will be the answer here, if its data coming directly from your hard drive then the problem you get is pretty different.
Some other things for you to check is the size of your virtual memory within windows.
Minimizing windows processes such as aero in vista.
Ensuring your hard drive is correctly defragmented.
Check over what your virus scanner is doing, some virus scanners have great security but greatly affect computer performance.
Ensure there are no other rogue items causing this issue.
Use PS2 input devices as opposed to USB input devices. If you have an external USB Hard drive this could slow down the overall bus speed thus affecting your response time on USB input devices. Also is your virus scanner constantly scanning your external drive?
I implore you check out these issues before potentially wasting money on a really expensive hard drive. By the way what are the general file sizes of your photoshop files?
EDIT:
Run your files from your Internal Hard Drive. The problem will be the swap rate, as opposed to passing files straight through your RAM the photoshop file is passed to your internal hard drive then your RAM (increasing the rate from 2 processes to potentially 5). This is because when you use an external hard drive it cannot fully use the DMA (Direct Memory Access) feature built into your mother board. The USB bus is on a seperate BUS hence the slow speeds. I recommend making changes to PS files completely on your local hard drive and then schedule a backup to occur at the end of each day. Therefore you have everything externally and the speed increase you desire.
The caching process you described is accurate, however if you think about it the files still need to interact with your OS, without getting too technical the basic cut of the story is that your external drive and direct access to that is causing an increased swap looping time. And making a mess of communications between all your setup. Ill go out on a limb and guarantee this will fix your problem if you dump the process of working directly off the external hard drive. Either that or get and external sata2 connection which runs directly from your sata2 controllor. As opposed to currently communicating through your USB bus - Sata bus - RAM bus - Processor - Then Graphics card*
*In some occasions
Oh and one last thing setting Photoshop to utilise all of your RAM could be causing this issue at the highest i recommend restricting photoshop to using only 70% of your total RAM after system processes are taken into calculation.
I wouldnt mess with cache settings this is only ideal when you have a raid configuration and your motherboard is capable of striping two streams of data. Read up and Direct Memory Access and how storage controllors interact with the elements of your motherboard if i have lost you somewhere.
Hope this helps