Stored Proc To Get Single Person From A Table Based On Earliest Datetime

Hi,

I'm having problems with a stored procedure, that i'm hoping someone can help me with.

I have a table with 2 columns - Username (varchar), LastAllocation (datetime)

The Username column will always have values, LastAllocation may have NULL values. Example

Username | LastAllocation
------------------------
Greg | 02 October 2005 15:30
John | 02 October 2005 18:00
Mike | <NULL>

My stored procedure needs to pull back a user name with the following criteria:

If any <NULL> dates send username of first person where date is null, sorted alphabetically, otherwise send username of person with earliest date from LastAllocation

Then update the LastAllocation column with GETDate() for that username.

This SP will be called repeatedly, so all users will eventually have a date, then will be cycled through from earliest date. I wrote an SP to do this, but it seems to be killing my server - the sp works, but I then can't view the values in the table in Enterprise Manager. SP is below - can anyone see what could be causing the problem, or have a better soln?
Thanks
Greg
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CREATE PROCEDURE STP_GetNextSalesPerson AS
DECLARE @NextSalesPerson varchar(100)

BEGIN TRAN

IF (SELECT COUNT(*) FROM REF_SalesTeam WHERE LeadLastAllocated IS NULL) > 0
BEGIN
SELECT TOP 1 @NextSalesPerson = eUserName FROM REF_SalesTeam WHERE LeadLastAllocated IS NULL ORDER BY eUserName ASC
END
ELSE
BEGIN
SELECT TOP 1 @NextSalesPerson = eUserName FROM REF_SalesTeam ORDER BY LeadLastAllocated ASC
END

SELECT @NextSalesPerson
UPDATE REF_SalesTeam SET LeadLastAllocated = GETDATE() WHERE eUserName = @NextSalesPerson


COMMIT TRAN
GO