When you declare all your queries as statics they will of course take up memory like that but you will have a reference to the same strings so it will be dead quick.
When you use the StringBuffer you will create a lot of strings anyways and they all get stored into the string pool anyways so in effect they will take up memory.
Using a stringbuilder or even stringbuffer which is slower will take more time in the long end. In my opinion there's no reason to use a StringBuilder if you know that the strings will be the exactly same every time.
Otherwise you can create a properties file in your web-inf/classes folder called databaseQueries.properties and load that into memory and just get the query you need from it.
The amount of memory taken up by your queries will be nothing compared to the objects you will be putting on the session anyways I believe.