Should I opt for a new database or adding more tables?
I would definitely err on the side of keeping the tables in the same database. It makes it easier to maintain referential integrity (unless you hard delete records in which case foreign keys won't work for you anyway) and create consistent backups (simply backup one database and you know it is in a consistent state, instead of taking two+ backups that my be slightly out of sync).

If you concern is wanting to spread the data between different storage pools (for instance keeping history and audit data on something big &amp; cheap but potentially slow, while keeping active data on something faster but smaller &amp; more expensive), or just keeping the different data on different spindle-sets to reduce potential IO contention, then you can achieve this within a single database by distributing your tables between files/filegroups instead of just having one filegroup containing everything. See http://technet.microsoft.com/en-us/library/ms189126(v=sql.105).aspx (amongst other reference locations in MS's sites) for more details on this.