You really need to look at each DB separately as to resource usage and maybe size.
Then you have to ask which consolidation method is best for the DB.
If the DBs are similar in nature and function, then consolidating into a single DB, perhaps using schemas, may be best.
Other factors, including security, that are kind of beyond the scope of a quick forum answer can result in consilidating multiple DBs on a single instance, creating multiple instances for each DB, or set of DBs, or using virtualization.
This is an article that gives a pretty decent overview: http://technet.microsoft.com/en-us/library/ee819082(SQL.100).aspx