I use Thule load bars. I recommend the Thule 450 Crossroad feet (which attach to the BMW rails) and Thule LB58 load bars, which are 58 inches long. Consider adding a set of 6 locks so that you have a single key and the rack stays attached to your vehicle.

You can get away with shorter bars (I have a set of 50" bars from a previous vehicle that I have used) but they will only just reach across. It is a cleaner installation, but you won't be able to attach carriers as easily near the edge of the vehicle, as there is no load bar sticking out from the attachment feet when they are mounted on an E53 X5.

There is a fancier Thule load bar available (rapid), with different feet, but I don't see the point unless you want to quickly change carriers on the load bars.

There are also wind deflectors available, but I haven't used them.

You can build this up on thule.com, and see their list of dealers. Many auto parts places carry Thule.

WRT installation, the above parts assemble without tools. 10 minute job once you have done it once. You will need to attach the box to the load bars.

Good luck.

__________________
2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue