one or more named data cache for performance

Suppose I have a DB with size 20G(include data and indexes). I have enough memory say 30G.

Then I can create named data cache to cache the whole database.

Here is my thinking:

One 20G cache the cache whone DB.

Two caches: one is for data(bound it to DB), another cache index(bound to all indexes)

Which one is better? I am thinking use one for all, but not sure.

then extra 10G I can use for log, tempdb, Procedure cache, overhead, ..., right?

Also, how can I know if the cache is full?