Hi shigadeyo,
Congratulations!
There is more detailed information about toppers and the posts linked to therein (included resources for manufacturers offering toppers), in post #2 here.
The added comfort that a topper is able to provide is of course impacted by the surface upon which it is placed, your sleeping style, somatotype and other personal preferences and sensitivities.
Talalay latex is a popular choice for toppers, often in 2" and 3" thicknesses for added plushness. The ILD range of 15-19 is often considered more “ultra plush”, and the 20-24 ILD range is often considered “plush”. If your existing mattress is already quite soft and your wife desires just a little extra comfort, a 2" toper might be adequate. If your wife needs more substantial comfort and your existing mattress is firmer, the 3" topper might be a more appropriate choice.
I hope that information helps out.
Phoenix