thats one of the most open questions..

and obviously it depends on what you plan on using the vehicle for..

if your talking about performance driving you can get sways, struts, stiffer suspension, lowering the car for lower centre of gravity, high performance tyres.. sky's the limit..

but then again why would you want to..its an x5..not a sports car.. not to say that it isnt fun to take for a squirt around the twisties..