With the $400 budget you might be able to get Intuos A5 (Wireless is not that much more expensive than the normal version..) Intuos is awesome device.
( By the way.. Wacom devices usually come with Corel Painter or Photoshop elements which is a good thing to know and keep in mind. )
Then again, new bamboos have separate wireless adapter that you can buy around $60.
The new bamboos seem to still have "paper like surface" which is total crap if you ask me. I'm not saying that it is not like a paper surface, but the existence of paper like surface or any grainy surface for drawing tablets is just a bad choice as far as the pen nib's go if you ask me. (It could be a lot less grainy than it was before in the earlier generation bamboo's, but I would always go for the smooth if possible..)
Intuos has smooth smooth surface and you can buy different overlays for them. Like, transparent overlays that you can buy separately allows you to, put paper drawing under the overlay and basically use that to re-draw your older drawing.. Personally, when I had Intuos and the clear overlay, I never used that.. With Intuos you can use one pen nib for a long long time Especially the "Stroke nibs". "Standard nibs" wear out a bit faster, but still last quite long and my personal experience with "Felt nibs" is that they wear out really fast even with smooth surface.
Though as a third option I would throw in Inkling while keeping in mind that you can't color with it. Inkling is around $190 It's not super accurate and is meant for sketching mostly. So, it's a ball point pen with real ink that you can use to draw on paper. It has this receiver that you place on top of a paper and it records the drawing digitally. Works with Photoshop cs3+ and Illustrator cs3+. I heard that you are able to do some computer controlling with the pen, but I wouldn't really count on that much. I personally am planning to use it accompanied with my other tablets.. (when my inkling arrives) So, it might not be the right choice if you need to color the drawings.. Though some general coloring isn't so hard to do with mouse either.
The size can be pretty important thing. Generally I would say, buy tablet that has drawing area about the same size as your normal drawing area. It takes less getting used to.
As far as actual drawing goes, I wouldn't get A6.. for painting I would still use that. Personally I like A4 size. Which I think is quite ideal for most. A5 or often called Medium, can be quite good size especially if you have a budget and I would still use that for drawing.
So, basically Bamboo or Intuos, you can't make a wrong decision as far as the hardware goes.
But I would definitely take Intuos. Especially if you're not feeling crafty.. (Read more below..)
If your wife is more used to traditional drawing techniques you might want to buy tablet that comes with Corel Painter.. (It has a large variety of different tools that imitate quite well the real life version of the tool.. like different paints and charcoal.. pencil.. etc..)
Pen nibs wearing out really fast is definitely a problem with the bamboo line (I don't know for sure if all of the older generation one's have grainy surface, but bamboo fun does.) It doesn't have smooth surface like Intuos has. I used 1 pen nib all the way in 2 days and almost didn't get it out afterwards cause it was so worn out.
But, the crafty guy that I am.. I bought thin transparent double sided tape and transparent overhead projector overlays (transparency doesn't really matter) and made my own smooth surface on top of the bamboo's own grainy surface. Now I've had one pen nib in use for about a year and you can just about see that it has worn a little.
I have to change the overlay at times cause it sort of stretches and next time I probably try printer overlays which are a bit thicker.