I run basically the same thing as this. Mine is actually an old coralife unit Snappy gave me.
http://www.bulkreefsupply.com/store/...my-system.html
I have not ever felt a need for more stages than this. I flush my membrane religiously each time and only allow water to enter the resin when it's 3 TDS or less. Filters and resin last me a long time.