Well here are my opinions. I urge you to talk to someone else to double check my opinions because by trade I am not a designer, however I have supported graphic design departments for several years.
First off, don't buy a PPC mac. That means no G4s, G5s, etc. Get an intel Mac. The main reason is that CS3 was designed to run on an Intel Macintosh computer, and that is the future of that company. It also makes it more compatible for windows users. If, your wife has this one little application she has to run and its windows only, you can run windows on an Intel based mac. So, it basically breaks the application barrier that some users have. Also, since you use a PC and you want to share applications (like you own a PC version of something she wants to use) it is also possible by either loading windows on the Mac or running a virtual machine.
Now, here is the hardest part, deciding what mac you want. To be completely honest with the Mac Pro is overkill for almost every user. I mean for one it runs off of Xeon processors, which are Intel's work horse processors, and its also the same processors that servers use. If she does some serious photo editing, or serious video and audio editing, then yes the Mac Pro is worth the investment. If she does professional grade photo editing but doesn't quite hit the "hollywood" level production, then perhaps an iMac will do fine. It really depends on how she uses it. I mean when it comes to pure number crunching power the Xeon will kick the crap out of Core 2 duo every time. When it comes to multimedia play back, like audio/video there is no difference, and video games would run better on the Core processors.
Data storage is something that can be simply done by just building or using an old/cheap PC and setting up a file server. You don't need something powerful and you don't need to upgrade your current machines. You can also centralize data in this aspect and then back it up again for redundancy. For example, I have built cheap (like $300) PCs and slapped large HDs in them, then loaded Linux (a free OS) and set up file shares. Which can be mapped to both Windows and OS X machines. Of course this is just one way of doing it. Adding in internal drives or getting external FW drives are other ways of doing it. The plus side is its a bit cheaper to just buy a drive, but the down side is that it is harder to organize. So, if you need to make sure you have back ups of your current data, centralizing it makes it easier. If you have an old PC lying around you aren't using, you could use that.
If your wife needs mobility Macbook pro would be the way to go. She could then bring her photo work to her clients directly, and load digital photos onto your mac on site. Then set up a file server at home or her office so she can regularly back up her data.
I once did data recovery for a recording studio that had a crash, first crash they ever had and the Mac was 8 years old. No back ups, and there were like 3 or 4 almost finished albums on this rig. It took me many hours and I was able to salvage most of the data, but some of it was lost. Needless to day, the clients of that studio were pretty pissed off. Your data is the most important thing on your computer when its your business.
Me personally, I have owned, repaired and maintained a HP laptop, Sony, Compaq, 2 different ibooks, Acer, gateway, and several others (I didn't own all of those, but I had to deal with them at work) and I can safely say that my Macbook Pro is by far the best damn laptop I have ever owned. It is fast, reliable, dependable, sleek, cool, and has every feature I would ever need right out of the box. I also run 3 OSes on it and use it daily at work. For me, I need the Macbook Pro to do my job. For your wife, I would have her seriously think of all her options, because I think for photography have the ability to be mobile is a huge plus. Also, buy the extended warranty because Apple includes phone support with it, so you can call them for support as many times as you want. Where as with Microsoft they charge you per a support call. Apple will charge you too, but if you have apple care that is an included feature, just FYI.
