The new xbox (slim) has a built in power supply for project natal/kinect, where as the old (current xbox) would require a sepearate power supply.
Also the new xbox has a built in 250gb hard drive (available sepeartly for old machines), and a Wireless N adaptor built in.
So there are some benefits for the new console.
The console launches end of this week coming. Project Natal/ Kinect launches Novemeber, xbox have NOT as of yet announced pricing for it, although the internet has it at 100 plus pounds. (Microsoft have not said how much it will be, the whole £100 pricing has been established for a price that MS put out on the web as a guide) Games for kinect will cost in the region of £40 also.