In general, importing a style that has an associated style does not overwrite an existing style of the same name in the target drawing. I have not tested that extensively with style-based PSDs, however, and given that there could be differences in any manual properties included in the two definitions, just using the local definition could cause data loss. This may be an occasion where "extra" copies of the same PSD get created [with numerals in parentheses at the end to create a unique name]. Not that you really want that to happen, either, as a schedule tag or table will only pull data from one of the PSDs.
I would agree with galingula that any custom PSDs you create should have their own, unique names. Yes, that does mean customizing the schedule tags and schedule table styles to use that PSD name, but in the long run that is worthwhile, if for no other reason than to identify styles/definitions that have been customized and need to be backed up and brought forward to new releases. If you find keeping the same name more to your liking and workflow, then you may want to copy your PSDs into the ADT 2008 source files for objects that have attached PSDs for which you have a custom version. At the very least, I would back up the out-of-the-box source files before doing that, and I would recommend spending some time checking the 2008 out-of-the-box PSDs against your 2007 versions, just to see if there are any new things added that you may want to have in your version, too.
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David Koch
Autodesk Discussion Group Facilitator
Using ADT 2004 at work; access to 2005, 2006 & 2007 at home