For us consumers. you can't go wrong with either Nikon or Canon in my mind...and I'd also throw in any of the other true camera companies like Pentax, Olympus, Sigma etc. Nikon and Canon have the biggest share of mind hence they seem to have the most lenses and all 3rd party companies seem to make lenses for Canon or Nikon mounts. Pentax mounts also seem to be widely available and Sony seems to be common now too.
As to your questions:
Rebel - a number of us on the board have the XTi and we all seem favourable to it. I think the XSi is a bit overkill and from what I've seen got a lot of "meh" reviews. The Canon XT is also a very good camera which you might be able to pick up cheap on ebay.
Comparable Nikon - I believe the XTi was to compete with the D80. The D40 and even the new D60 would also be comparable I think.
Lenses - The mount on lenses are different for each camera manufacturer so you'd have to get nikon mount lenses if she switched.
Again, I really doubt for us amateurs it makes much of a difference which company's badge is on the body, I'd say it more comes down to personal preference.
I've had a couple of Canons so I was familiar with their menu system, so I was leaning that way. Also I wanted a light system for travel purposes so the XTi was the direction I went. Also I don't have big mitts so the XTi fit my hand better...this is something you might want to consider since it's for your wife.
Best is to walk into one of the camera stores and have her try out a few models so she can pick the one she's most comfortable with. Also heed Neeper's advice on lenses, they are as he says more important.
Personally I have a Sigma 18-200mm IS lens which sits permanently on my XTi. It gives me the range I need for what I use this camera and has very good quality glass. If I need longer distances - like my recent trip to Tanzania - I have an ultrazoom camera and tele converter. (having 750mm worth of lens that weighed next to nothing was of huge convenience when on that trip).
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