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DBCC CHECKDB WITH DATA_PURITY has nothing to do with CHECKSUM. DATA_PURITY will check the values stored in the columns are valid for the data type. For databases created in SQL Server 2005 and above, these are done automatically when you run CHECKDB but for the databases created earlier you have to explicitly flip the bit by running the DATA_PURITY switch in the DBCC CHECKDB command.

When you enable PAGE_CHECKSUM, it doesn't really go after all the data pages and write the checksum. There is no command like that to write the CHECKSUM in all of the pages. One way is to rebuild all indexes and this will cover only the CI, I but heaps are NOT touched. For heaps, you may have to do an in-place update like below.

Update heap set c1 = c1

Disabling CHECKSUM to get some performance benefit is NOT a good idea. There are so many other things you can do that will get lot more bang for the time spent. Please don't do that.

On this related topic, I did some tests on the impact of enabling BACKUP CHECKSUM and teh results were very interesting. I did ask MSFT team on why would this happen and didn't get an answer so far. So, my take away point here, always test even if you happen to know teh answer and take action on the first hand information.

http://sankarreddy.com/2010/03/performance-impact-of-using-backup-checksum-in-sql-server-20052008/[/url][/font]