Here's a quick test I threw together to see what happens:

IF OBJECT_ID('dbo.KeyTest', 'U') IS NOT NULL

BEGIN;

DROP TABLE dbo.KeyTest;

END;

CREATE TABLE KeyTest

(

ID INT NOT NULL

IDENTITY(1, 1),

OtherDate CHAR(500) DEFAULT 'A'

);

CREATE CLUSTERED INDEX CX_KeyTest ON dbo.KeyTest(ID);

GO

INSERT INTO dbo.KeyTest

(

OtherDate

)

SELECT TOP 500

AC.name

FROM

sys.all_columns AS AC;

GO

SET STATISTICS IO ON;

SELECT

KT.ID

FROM

dbo.KeyTest AS KT;

SELECT

*

FROM

dbo.KeyTest AS KT;

SET STATISTICS IO OFF;

GO

CREATE UNIQUE NONCLUSTERED INDEX UX_KeyTest ON dbo.KeyTest(ID);

GO

SET STATISTICS IO ON;

SELECT

KT.ID

FROM

dbo.KeyTest AS KT;

SELECT

*

FROM

dbo.KeyTest AS KT WITH (INDEX = UX_KeyTest);

SET STATISTICS IO OFF;

GO

SELECT

OBJECT_NAME(i.object_id) AS TableName,

i.name AS IndexName,

i.index_id AS IndexID,

SUM(a.total_pages) AS total_pages,

SUM(a.used_pages) AS used_pages,

SUM(a.data_pages) AS data_pages,

8 * SUM(a.used_pages) AS 'Indexsize(KB)'

FROM

sys.indexes AS i

JOIN sys.partitions AS p

ON p.object_id = i.object_id AND

p.index_id = i.index_id

JOIN sys.allocation_units AS a

ON a.container_id = p.partition_id

WHERE

i.object_id = OBJECT_ID('dbo.KeyTest', 'U')

GROUP BY

i.object_id,

i.index_id,

i.name

ORDER BY

OBJECT_NAME(i.object_id),

i.index_id;

GO

CREATE UNIQUE CLUSTERED INDEX CX_KeyTest ON dbo.KeyTest(ID) WITH DROP_EXISTING;

GO

ALTER INDEX UX_KeyTest ON dbo.KeyTest DISABLE;

GO

SET STATISTICS IO ON;

SELECT

KT.ID

FROM

dbo.KeyTest AS KT;

SELECT

*

FROM

dbo.KeyTest AS KT;

SET STATISTICS IO OFF;

GO

SELECT

OBJECT_NAME(i.object_id) AS TableName,

i.name AS IndexName,

i.index_id AS IndexID,

SUM(a.total_pages) AS total_pages,

SUM(a.used_pages) AS used_pages,

SUM(a.data_pages) AS data_pages,

8 * SUM(a.used_pages) AS 'Indexsize(KB)'

FROM

sys.indexes AS i

JOIN sys.partitions AS p

ON p.object_id = i.object_id AND

p.index_id = i.index_id

JOIN sys.allocation_units AS a

ON a.container_id = p.partition_id

WHERE

i.object_id = OBJECT_ID('dbo.KeyTest', 'U')

GROUP BY

i.object_id,

i.index_id,

i.name

ORDER BY

OBJECT_NAME(i.object_id),

i.index_id;

GO

To summarize, if just returning the key column, the non-clustered index does fewer reads, but as soon as you add the second column, if the non-clustered index is used it requires more reads due to the Bookmark/Key lookup. You also have to take into account the maintenance that having both indexes adds to inserts/updates/deletes. That's why I like using either Jason Strate's index analysis script[/url] or Brent Ozar Unlimited's sp_BlitxIndex both of which do some calculations to show the costs of indexes.