I have a g5 1.8 dual mac with 1.5GB RAM expandable upto 8GB RAM
expansion, PCI-X slots, 4x DVD drive, 160GB 7200 RPM HD, Photoshop 7,
latest OS X before Tiger and have just ordered a Raptor 72GB 10,000
RPM HD (not arrived yet).  I bought it for colour correcting (in
Photoshop 7) 20 weddings a year which I photograph.  A wedding
consists of approx 1200 images shot at on average 3MB per image ie 4GB
per wedding.  After shooting, I have about 6-8 hrs to go through all
the shots, usually
10 at a time in Photoshop, culling, cropping & colour/contrast
correcting.  I'd like to optimise my G5 so
it can (a) open 10 shots at a time as quickly as possible, (b) colour correct,
rotate, crop and enhance shots as quickly as possible, and then save
and delete as quickly as possible.  I have ordered a Raptor SATA
10,000 RPM 72Gb drive to help
speed up image load times.  WhatI'd like to know is how else I can
optmise to be able to process these pics as quicky as humanly
possible.  ie I have 1.5 GB RAM - should I up this?  I have the
graphics card that came with the machine 64MB 5200?  Should I upgrade
this?  Basically - what are my weak points to get maxiumum load, edit
and save speeds so the only limiting factor is my edit decisions
rather the whirring of hard drives, or the slowness of the machine?  
I also am starting to do video editing - iMovie now, but starting
using Final Cut Express.  Typically using miniDV but increasingly this
year doing 5-10 minute of footage edit jobs into 30-60 sec bite for
group public speaking presentations - if changes to optimise photoshop
experience would improve FCE experience, then that would be a bonus. 
Is there anything else I shd think about adding to my setup too?  By
the way - I'd like to spend the money necessary for a good
improvement, but need to stay commercially viable - so ideally not
spend more than $300 unless it will have a large impact eg cut edit
time down by 2 hours.

I would upgrade to Tiger and use Automator to do most of the job for you

thanks for the suggestion, but you're missing the point.  Editing is a
manual process - the thing I want to speed up is the time to load
files (ie Raptor 10k RPM faster hard drive), speed up the rendering of
the images in PS (this I don't know how to do - more RAM, faster
graphics card or unable to speed up?) and then the save process (ditto
re hard drive).

When you're going through 1200 shots, 10 at a time, and waiting 20+
seconds per 10 shots to load, it adds up to 40 min wasted just waiting
to load pics.  This is what I want to address.  Ditto the edit time -
again making it as fast as possible, so the only delay to the whole
thing is my manual decision making time of figuring what each shot
needs re colour correction, convert to b&w, adjusting contrast,
doddging/burning etc.   The challenge I have is turning round the
whole edit in a night, or 6hrs tops.

Add more RAM - as this will be bogged down by opening and closing many
photos. Or buy a new Powermac G5 dual 2.7Ghz for the quicker
processor!

I don't think either of these are going to come in at less than 300USD.

Using a faster hard drive is a good start though.

If you're editing each picture individually then you would have to
work at a pace of one picture every 20 seconds for the 6 hour time
frame - which would seem unlikely. I think you're just going to
allocate more time to editing.

As fast as a Raptor is, it is still not that fast.  I have used that
drive and the real world max sustained speed on that drive is still
only  about 55MB/s.  That is slow as molasses when compared to your
RAM.

Buy another 2GB of RAM and create a 1.5GB RAM drive using this program:
http://www.clarkwood.com/rambunctious/ 

Copy 1GB of photos to the RAM drive in bulk, do something else while
waiting.  5 minutes of wait time is way better than 20 seconds every
few minutes.

Open and edit the photos in Photoshop from the RAM disk, when finished
editing, copy the folder back to the Raptor.  You may want to create a
script to save the RAM disk contents on occasion in the event of a
system crash.  However, you may find the setup to be perfectly stable.

When using a RAM disk on my laptop, my xBench Disk Score goes from 67
to 247.  You will have an even greater result with the G5 memory
architecture.

You can buy 2GB of Memory for <$300

a better grafics card will help your screen refresh the image.
when i upgraded from the integrated graphics card to a regular NiVida
card things speed up a bit.
also ram can help but i think that photoshop only addresses upto 2.5
gb of ram and that was just uped for photoshop cs2 so you might want
to find out how much ram your photoshop will address.
asking in the adobe photoshop forums will help if you cant find it online.
good luck