Cast NTEXT to INT for calculation and comparison
See comment by Martin Smith and https://stackoverflow.com/q/2133946/327165 for overview of why not to use ntext.

In order to get numeric records only, including records with numbers that have quotation marks try this: SELECT REPLACE(CAST(TextValue AS NVARCHAR(MAX)),'''','') FROM Table a WHERE ISNUMERIC(CAST(a.[Value1] AS NVARCHAR(MAX))) = 1