You are assuming the newb want to "learn photography".
Of course that is my assumption.... why else would we even be discussing this topic otherwise. For what else reason why this topic would be on "thephotoforum"? Beginner = someone who strives to be a good photographer not just someone with a nice camera.
When my wife wanted a "better camera", if she had seen pictures from you guys here ... she would have bought the 5D with whatever lens you guys used for that incredible picture. I'm sure the 5D with the 24-70 f/2.8L will take better pictures then the SD800IS which I gave her - even in green box mode mode

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Actually the Canon 5D with the 24-70 f/2.8L is about the WORST camera for your wife.
1) The 5D is lacking any "creative" modes. It only has the "core" Manual, shutter, aperture, program modes. The lower end models would best suited for her as she can choose between portrait, landscape, action, etc.. and the camera will select the best shutter/aperture combinations given the selected mode. On the 5D, one would need at least the fundamentals in order to leverage the camera.
2) 5D+24-70L is a very heavy setup. For the typical snapshooter, a heavy camera will equate to one that is left at home often. This equates to a very bad investment and experience.
3) Try explaining to her why she'll spend more money on a zoom of 24-70 (pretty darn short) when a MUCH cheaper P&S will have an effective zoom range of around 24-200mm. Canon G9 for example is equivalent 35-210mm with a measly 6x zoom.
4) Try explaining to her why a 5D costing several times more than the Rebel 450D still has the same number of Megapixels. Try to explain to her that the 5D+24-70 is worth the premium cost over the G9.
I've been on the other side of the counter enough to know that I can easily sell a high end expensive DSLR+lens setup to a enthusiast beginner. However... a snapshooter (like your wife) is actually a tougher sell for the high-end expensive DSLR+lens. Why? because their intentions are different. One strives to be "just like the pros" the later just wants good pictures.
In other words....
I can hand your wife a Rebel or a G9 and I guarantee that she'll by far be happier with the resulting photos.
That is $2500 in hands of a newb who will never have it off green box, will constantly complain about having to look through eyepiece, but will be perfectly happy because the camera allows her to take pictures of daughter running around in low light.
Successfully shooting a camera in low-light means the person will have to have a basic understanding of:
* slowest shutterspeed he/she can handhold given their steady hand and focal length.
* How shooting a lens at wide open will result in a very shallow depth of field and how to work around it.
* how to leverage existing lighting.
A newb will be far more successful using standard camera+lens with the appropriate flashpack. Shoot in low light on full auto "green box" without flash.. it doesn't work very well. For starters, AF is more useable once a dedicated flash is attached and emits a focus assist "beam".
Buddy.. you just reinforced my statements in almost every way.