The CPU at 100% is only a problem if it stays at 100% for a prolonged period of time. More important is disk IO and memory usage. If these are high then you will need to do some tuning of the databases, such as getting better hardware of splitting the databases across servers.
The next action to make is to run profiler ( http://msdn.microsoft.com/en-us/library/ms181091.aspx ). You could do a couple of passes with this. Firstly you could just look at connections to see which DB's are being used the most and which are not used. Next look for long running queries - these are the ones to tune as they will be using the most resources. If the database is transactional, look for the queries that are run very frequently (put the profiler output into a database table and count by query) and look at tuning them.
You could also explore the system tables which store some of the information you need ( http://msdn.microsoft.com/en-us/library/aa260604%28v=sql.80%29.aspx )
If performance is still an issue, then you need to look at new hardware (bigger server, faster disk, more memory, faster network) or to split the databases across servers.