SQL Server Index Fragmentation
Rebuilding an index takes more CPU than reorganizing it.  It locks the database so that has to taken into account.  Indexes should be rebuilt when the fragmentation is more than 40% or so.  After that it becomes too slow and cumbersome for the server to reorganize.  You should reorganize an index when the fragmentation is roughly 10%-40%.  So healthy index is basically less than 10% fragmentation, little bit less than 10% perhaps.  

These are my rules of thumb and they have worked for me, but they are guidelines.  So many things change the performance so you need to find out what woorks for you in your environment.

I think that you can rebuild an index online nowadays, at least if you use the Enterprise version of SQL Server, the keyword ONLINE if I am not mistaken.