In terms of the cockpit differences to the current -400...
The displays are now LCDs instead of CRTS, there is a new collins whizbang multi-scan radar and the standby instruments are glass instead of analogue. The cockpit also has additional soundproofing. It has a terrain display for the ND (don't fly into the red bits), but some existing -400s already have this.
Do a search (also try dunnunda and godzone) to find out the technical details of the extra centre wing tank, beefed up landing gear etc.
There are many minor differences. One I noticed is that the cargo fire suppression capability has been increased from 195 minutes to 215, I guess because it will be spending longer in the air.