3.0si: no arches, does not come with 20" wheels, though they can be added yourself
4.8i: you need the upgraded wheel pkg to get the arches and 20" wheels

There are other cosmetic differences like the kidney grills and tailpipes. Mechanically obviously the motor. Also note the '07s might not have the moonroof as it was an option whereas it was pretty much a given on the '08s for either model. '07s didn't have auto-tailgate or auto-hold either.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr