To add a few links/thoughts...
The forum for them is down now...There is a thread on building a machine for C1Pro.
Faster processor makes faster tiff files from Raw files, not ram or hard drives ( to a point)
Faster hard drives make faster thumbnails when shooting tethered.
The Phase tech wrote that ram isn't really an issue, like 2 gigs is fine.
Now that is for C1, not photoshop, photoshop likes ram.
Look into "Ram disks" and using more than 3 gigs of ram as there are still issues. I have read somewhere that you can make a ram disk, mount it and use it as a scratch for PS, this isn't as automatic as it seems.
Oh, looking at running a lot of actions. SET history to 1 state and cache to 4 and RESTART Photoshop and then run actions. Makes a big difference.
For Fun run this speed tests on what you have now.http://retouchartists.com/pages/speedtest.html
Here is a very fast drive, be carful which version you get as there are issues with fitting it in a Mac Pro...READ the WHOLE ARTICLE. They now sell the drive with a different heat sink, no heat sink and standard heat sink. http://www.barefeats.com/hard103.html
and read the update about sleds/fins/bare drives.http://www.barefeats.com/quick.html
Raiding 2 drives to act as one can be very fast but dangerous as if you loose one you loose everything, but as stated above with Time Machine you are covered.
If you need to restore from TM by the way, you need to put in a new HD THEN in stall the OS and then restore from TM.
Also take a look at "SuperDuper!" in google, if your machine dies you can just boot from the SuperDuper drive and keep working, you don't lose 4 hours rebuilding your system but it isn't as sexy as TM.
I have also heard that custom configuring the Pro with the upgraded graphics card for $150 is a good idea.
I hope this helps.