Hi Brownie19,
The only real difference between a “posture board” or bunkie board is that they have a lower profile than a rigid slatted foundation. Generally you will find regular profile foundations (typically 8-10"), low profile foundations (typically around 5") and bunkie boards (typically about 2"). The only real difference between them is that they provide a way to raise or lower your mattress higher or lower on your frame. In some cases such as if you have a slatted bedframe, you won’t need a foundation at all as long as the height of the mattress is acceptable to you.
There is more about bunkie boards in post #4 here and there is more about the many different types of foundations available in the foundation thread here.
If you are looking at a mattress that has a foam base (either polyfoam or latex) … then a slatted rigid foundation is usually the best choice yes. If the base foam is latex then the gaps in the slats should be 3" or less (or have a wire grid surface which provides even and strong support for the mattress). If the base foam is polyfoam then wider gaps between the slats or grid is fine (following the recommendations of the mattress manufacturer).
A good retailer would be able to tell you the density/quality of the support foam in the base (or call the factory to find out if they don’t know). Generally 1.8 lbs is the minimum density I would consider in a good quality mattress (and I believe this is what it has but it could vary depending on which licensee makes it). Lower than this would be found more (or at least should be) in a lower budget mattress. Higher density would be better yet but 1.8 is the “minimum” I would look at outside of lower budget mattresses…
Phoenix