Typically, rebuilds should be run when you can have exlcusive use of the database. It will impact the server performance, it will log to the transaction log, use tempdb and, I think, you should have like 50% of the amount your database size free with respect to disk space for that database (I may be wrong about this number, it may be more!). Defrag can be run with users in the system but again, there will be a performance hit. For index rebuilds, there a plenty of scripts on this site that will allow you to do a selective rebuild, i.e. if the index is x% fragmented, rebuild it, look around if this fits your needs. This would allow for a quicker, possibly smaller, rebuild process.