Can I use replication/mirroring to have a "Current" database and a "Full History" copy?
Define the data is getting to be a bit much?  I've got clients with 1B+ rows in a table and no performance problems at all.

You could use SQL Server replication to replicate inserts and updates to a remote machine, but basically have it ignore the deletes.

Before I'd start looking at hacking together something like that I'd probably look at index tuning or query rewriting to see what I can do to resolve the performance problems with the queries that I've got.