Question About Transaction Locks And Isolation Levels

Have the need for going to a table to get an identity value. This is for updating an existing database, blah blah blah. Here is the schema of the table we are using:CREATE TABLE [TableIdentityValue] ( [TableName] [varchar] (50) , [NextNegativeIdentity] [int] NOT NULL , [NextPositiveIdentity] [int] NOT NULL , CONSTRAINT [PK_TableIdentityValue] PRIMARY KEY  CLUSTERED  (  [TableName] )  ON [PRIMARY] ) ON [PRIMARY]GO

Now, depending on the type of data we are inserting into a table, we need to get either a negative or positive number for the PK. There are two sprocs that control the obtaining of those values:CREATE PROCEDURE GetNegativeIdentity @tableName varchar(50)AS DECLARE @nextNegativeIdentityValue int

 BEGIN TRANSACTION SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

 SET @nextNegativeIdentityValue =  (  SELECT NextNegativeIdentity  FROM TableIdentityValue WITH (ROWLOCK)  WHERE TableName = @tableName )

 UPDATE TableIdentityValue SET NextNegativeIdentity = @nextNegativeIdentityValue - 1 WHERE TableName = @tableName

 COMMIT TRANSACTION RETURN @nextNegativeIdentityValueGOCREATE PROCEDURE GetPositiveIdentity @tableName varchar(50)AS DECLARE @nextPositiveIdentityValue int

 BEGIN TRANSACTION SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

 SET @nextPositiveIdentityValue =  (  SELECT NextPositiveIdentity  FROM TableIdentityValue  WHERE TableName = @tableName )

 UPDATE TableIdentityValue SET NextPositiveIdentity = @nextPositiveIdentityValue + 1 WHERE TableName = @tableName

 COMMIT TRANSACTION RETURN @nextPositiveIdentityValueGOSo, the thing is, we need the read and update of the value from the specific TableIdentityValue row to be atomic - we don't want anyone else reading or modifying that data. The problem is knowing which level of isolation to use and/or locking, and how to implement that. I have tried a few different things that seemed to make sense, like placing a ROWLOCK on the SELECT statement, but is that lock going to hold for the entire length of the transaction? Also, I read that using some of the lock hints can be accomplished in the sense that some isolation levels are the same as some lock hints (e.g. setting isolation level to SERIALIZABLE "has the same effect as setting HOLDLOCK on all tables in all SELECT statements in a transaction" according to SQL Books Online.Any help is appreciated!