My plan is/was to create an object
modeling the spreadsheet, parse the
data into it and use LINQ to do some
checks client side before sending the
data to the server... Basically I need
to compare the data in the sheet to a
matching record (Unless none exist,
then its new). If any of the fields
have been altered I want to send the
update.
You probably need to pick a "centricity" for your approach - i.e. data-centric or object-centric.
I would probably model the data appropriately first. This is because relational databases (or even non-normalized models represented in relational databases) will often outlive client tools/libraries applications. I would probably start trying to model in a normal form and think about the triggers to maintain audit/history as you mention during this time also.
I would typically then think of the data coming in (not an object model or an entity, really). So then I focus on the format and semantics of the inputs and see if there is misfit in my data model - perhaps there were assumptions in my data model which were incorrect. Yes, I'm not thinking of making an object model which validates the spreadsheet even though spreadsheets are notoriously fickle input sources. Like Remus, I would simply use SSIS to bring it in - perhaps to a staging table and then some more validation before applying it to production tables with some T-SQL.
Then I would think about a client tool which had an object model based on my good solid data model.
Alternatively, the object approach would mean modeling the spreadsheet, but also an object model which needs to be persisted to the database - and perhaps you now have two object models (spreadsheet and full business domain) and database model (storage persistence), if the spreadsheet object model is not as complete as the system's business domain object model.
I can think of an example where I had a throwaway external object model kind of like this. It read a "master file" which was a layout file describing an input file. This object model allowed the program to build SSIS packages (and BCP and SQL scripts) to import/export/do other operations on these files. Effectively it was a throwaway object model - it was not used as the actual model for the data in the rows or any kind of navigation between parent and child rows, etc., but simply an internal representation for internal purposes - it didn't necessarily correspond to a "domain" entity.