Stored Procs With Variable Number Of Parameters

I have received a change request for a project i am working on which im unsure if possible or not.

at the minute i have a very simple sp that simply selects data from a table by passing in one parameter

CREATE PROCEDURE PHAR_SelectGrade

@int_ID int

AS

 SELECT GradeID as ID, Grade as Description, RequiresText, SendEmail, Timestamp
 FROM tbl8Grade
 WHERE Obsolete=0
AND GradeID = @int_ID
ORDER BY Grade

the user now wants the option to select more than one grade type (there are 100's of different grades)

so they might decide they want to see details for 2 grades, 22 grades etc. these would be selected and stored in an array (via an asp.net project) and then the stored procedure is called from within a web service

my question is how would i pass this array into the stored procedure??

i am assuming i would need to do something as follows for sp syntax, but im stumped on how i pass my array of values into the sp from the webservice and then for the sp to read the array in SQL??

CREATE PROCEDURE PHAR_SelectGrade

@myArray <what datatype?>

AS

 SELECT GradeID as ID, Grade as Description, RequiresText, SendEmail, Timestamp
 FROM tbl8Grade
 WHERE Obsolete=0
AND GradeID IN (@myArray)
ORDER BY Grade


any ideas are greatly appreciated. or maybe its just not possible?!?

Cheers,
Craig