What is the difference between the Duemilanove and the Arduino Uno? Except for the price?
The type of USB-to-serial converter chip. Does it matter? Probably not.
Herefore i would need a wireless Xbee shield, is this correct?
An an XBee to put on the shield.
But do i need a receiving unit as well?
Yes.
And if yes, would i need 6 or can just 1 receiver work for all accelerometers?
You are connecting the accelerometers to the Arduino, not the XBee, aren't you? If so, then you need one XBee with shield per Arduino.
Also, would the Xbee requiere that there is a wifi connection available?
No. XBee is NOT WiFi.