UPDATE Concurrency?

I have a table where I count how many emails of a given type are sent out each day. This incrementing is wrapped in a sproc that either inserts a new row, or updates the existing row. The column that counts the value is named Count of type INT.

Below is the sproc, seems like a straightforward thing. However, I'm seeing email counts higher than they should be when there's a high number of concurrent executions of the sproc. I'm pretty sure it's not a problem in the calling code, so I'm wondering about the UPDATE statement, since it updates a column based on the value of the column. I would think this should work since it's wrapped in a SERIALIZABLE transaction, anybody have further insight?

SQL Server 2005 by the way.

Sean

CREATE PROCEDURE [dbo].[IncrementEmailCounter](
    @siteId SMALLINT,
    @messageType VARCHAR(20),
    @day SMALLDATETIME
) AS
BEGIN
    SET NOCOUNT ON;

    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
    BEGIN TRANSACTION

    IF (SELECT COUNT(*) FROM EmailCount WHERE SiteId = @siteId AND MessageType = @messageType AND [Day] = @day) = 0
        INSERT INTO EmailCount (SiteId, MessageType, [Day], [Count]) VALUES (@siteId, @messageType, @day, 1)
    ELSE
        UPDATE EmailCount SET [Count] = [Count] + 1 WHERE SiteId = @siteId AND MessageType = @messageType AND [Day] = @day

    COMMIT TRANSACTION
    SET TRANSACTION ISOLATION LEVEL READ COMMITTED
END