My two bits here -
I use presently an older Mac Pro. It is a quad 2.66 with 9 gigs of Ram and 3 drives internally striped (raided) together. My 4th hard drive is a backup drive and I use Superduper for the back up (silly name but nice program).
My software - Lightroom, Photoshop CS4, DXO, Capture One, LightZone and AI Silverfast.
So, given I handle large files too, I can tell you that the Mac Pro 8 cores would work very well for your wife. The key is configuration with appropriate scratch files (she'll understand this) and reducing bottle necks such as drive speed and available RAM. A neat trick is to create a "RAM Drive" for a swap file. If you want more info on this pm me.
As for Vista - (my turn to play on the bandwagon)..I started with early versions of DOS, Pharlap Extended 32 DOS, OS/2, all flavours of Windows, Novel, early Slackware, various Linux distributions. When at work it came time to review Vista as it was coming out - I went to MAC. This was a choice based on how "good" OSX is vs just how terrible Vista is (both 32 and 64 bit). I did this in the capacity of being a) Technologist for the Desktop Enterprise and later as a business analyst for the Information Security department. Please don't take my word on how bad Vista is, see the changes made for Windows 7 <grin>. Microsoft knows they had a LOSER with Vista.
Back to the point -
Word has it you can up the RAM on Macbooks to one of each - 2gig+4gig sticks of RAM and exploit 6 gigs. You may want to check that out. In order of performance you would have Mac Pro, iMAC and last would be the Macbook Pro. (assuming that the iMac processor is equal or greater and ditto for the video).
One of the things to look for is a good video card as you will probably take advantage of open GL in Photoshop. (A newer setting in CS4)
In your shoes I would get the Mac Pro 2.8, minimum RAM and minimum hard drive. Do get the better quality video card but not the very top of the line one. After that, consider purchasing RAM thru OWC
http://eshop.macsales.com/ and search around for larger hard drives on sale. Even Best Buy at times has 1 tb and 1.5 tb drives on great sales. The beauty of the Mac Pro is having 4 internal drive bays, ability to raid via software or add a raid card, additional slots for cards (like a PC) and easy access to RAM for upgrading. If your wife has to absolutely use a software in Vista, she can do it through Fusion or Parallels software and call it from inside of the OSX system. I have XP running in virtual mode on two machines without problems.
Last - I can't emphasize enough how configuration of Photoshop really does make a difference. Please check the internet on "optimizing for Photoshop" or some similar search.
As for me, as I have left for the most part IT world, I have for months been working doing digital photo restoration. I can at times can at 600 dpi or greater (depending on what will be done with the scan and the size of the original) and end up with large files in TIF format. The Quad 2.66 doesn't blink at all while working.
Hope this helps a bit.
- Phrehdd