Partitioning in SQL Server 2008
For what you want to do, I would recommend the following (which is pretty much what you were thinking).

1> Create history tables for the historic data you have - keep the schemas as similar as possible. Split up by some logical grouping (such as year/month) based on how they are going to be queried (say you need to report with in month/year as well as all). Do not worry about the table size of the splits unless they are getting into the TB size range (your dbms should handle it) just make sure that they are appropriately indexed for the queries that need to be run.  You should consider putting these onto a different disk to the active data if performance is an issue.

2> Create a routine to move data from the active table to the relevant historic table. Run this periodically. As a practice rebuild the indexes on the table that has had the data removed from it, and maybe update the table statistics. Easiest way to do this is to write a sql script.

3> Consider the reporting you want to do. If you want to only have to deal with 1 table when writing queries, create a view that joins the archived tables together. Create indexes on all the tables to suite the view. This way if you want all the data, select from the view. If you want data from a specific year/month, query that table. The view will look something like:


create view view_all_data as
select "Jan12" as month,a.* from data_Jan12 a
union
select "Feb12" as month,b.* from data_Feb12 b
....


I am assuming here that the system is not a highly used transactional system and that you have windows of low usage to run the analysis queries. If you need to maintain high levels of performance, you may like to do the above in a separate database (separate hardware) and port across the new data that you get from backups.