If your wife doesn't need intensive gaming or complicated environments and only uses a laptop for web browsing, office work or other rudimentary tasks then I would also say go for Linux to save money. You can buy a basic laptop for pittance or, if she doesn't mind the design, just install it on the mac.
I still choose windows because I'm a hardcore gamer and on that front, Linux still disappoints me.
If you want to buy a new windows laptop, here's what I did. First look at a competent company, as far as construction is concerned. Then look at the work space the laptop offers and then match specs which, as far as I can see, are not different from the mac.
I got a Toshiba Satellite L500 which is big and wide with a full keyboard, from a company that built my wife's still going eight year old machine. It has a decent CPU, GPU combo that can even run Crysis well. Perfect for my gaming needs.
If you only want one for office work, I'd go with a Lenovo laptop which are usually smaller, has a very comfortable design for long periods of work (I just love the joystick, they have) and they are also a very competent company (my brother has one which he has worked on for several years now). You don't need a very powerful CPU so just pick one of the Intel E brand or an AMD Athlon. And unless some high def video is involved, pretty much any GPU would do. I wouldn't go for less than 2GB of memory either way. And you might want to try breaking a 500GB HD in half and make it dual boot with Linux.
"They that can give up essential liberty to obtain a little temporary saftey deserve neither liberty not saftey." -- Benjamin Franklin, 1759