Compatiblity level in SQL 2005

What does are the rammifications in SQL 2005 of setting the compatibility level for a database to a lower level, for example SQL 2000 (80)?

Does this affect the underlying datastructure, the stored files on the server, or the indexes?

What effect does changing the compatibility level have? Does changing it cause the server to have to do any work to the indexs or table?

TIA!

refer this link,

ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/tsqlref9/html/508c686d-2bd4-41ba-8602-48ebca266659.htm

performance dashboard cannot be used if the compatability is not 90.......

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Compatibility level allows you to keep databases in SQL Server 2005 that remain compatible with prior versions of SQL Server. This also means that you cannot use Transact-SQL extensions introduced in SQL Server 2005 with a SQL Server 2000-compatible database. AFAIK there is not much change in storage architecture of sql server 2005 from 2000. Still they are stored in Pages/extents.

Madhu