I know this is an old thread but since it seems to be found often on google, I find it important to tell anybody reading this that our collective understanding of cxpackets was upgraded over the years. And by quickly skimming this thread I see some info that could use adjusting.

Paul White has spent a genormous amount of time writing this amazing article debunking that myth once and for all.

Have a nice read : http://www.simple-talk.com/sql/learn-sql-server/understanding-and-using-parallelism-in-sql-server/