Regarding memusage column I found this:
Pls note that BOL states that MEMUSAGE is only for backward compatibility and should not be relied upon.
The suggestion is to use perfmon counters such as what is listed below, agreed it does not answer your question in terms of finding out what table is in memory and which one is not.
SQL Server: Buffer Manager Object
Procedure Cache Pages In Use
Procedure Cache Size (pages)
SQL Server: Cache Manager Object
Procedure Cache Hit Ratio
Procedure Cache Pages
Procedure Cache Object Counts*
(http://sqlforums.windowsitpro.com/web/forum/messageview.aspx?catid=60&threadid=85706&enterthread=y)
Hope it answers your question.
Manu Jaidka