Splitting a database into multiple databases
If the problem is this single table getting huge, you should consider partitioning it (by date, since your data seems to be time-based).

This way the SQL engine query planner may be able to ignore some partitions (if they contain data too old to appear in your query results) when querying and thus perform better.

Additionally, partitionning will be handled transparently by your database engine, so you have nothing to change in your code.

Another advantage would be that you can imagine to move the 'old' partitions, containing data that is rarely accessed, to slower (and cheaper) storage.

You can also set up your indexes to be partitioned so you can only rebuild a part of an index:
http://msdn.microsoft.com/en-us/library/ms187526.aspx

Index partitioning is a different feature from table partitioning, it can avoid you to have to rebuild an entire index if it's not needed.