SQL Server 2005 - DataType of variable to store a RowSet

Question

In a typical stored procedure i am working there are various checks with the same query as below.

SELECT ... FROM Projects WHERE ProjectId IN (SELECT ProjectId FROM Tasks)

i would like to replace the query (SELECT ProjectId FROM Tasks) with a variable but am confused as to what datatype it has to be. Do you know? also caching this result is detrimental or is there easier way to doing this.