Thomas,
First off, with all the great enhancehancements and current high stability 
levels I would recommend going straight to version 6.4
As you have already stated there are certain data types hat are good candidates 
for data deduplication and your database backup data definitely is and image 
files definitely aren't.

>From my experience oracle export files are traditionally good dedupe 
>candidates also.
>From what you describe, the SQL backup data minus the compression would also 
>be a good candidate.

The one thing you do not mention is how many versions of this backup data you 
are keeping?

>From my experience, unless you are keeping a minimum of 4 backup versions, the 
>dedupe ratios will suffer. Too many time I see folks keeping only 2 backup 
>versions nd they can't understand why they get very poor dedup rates

Also be aware that with TSM deduplication you will have to ensure that you 
write the backup data to a target disk pool that  will have good enough 
performance to not negatively impact backup speed.

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