There are quite a few options unless you need to stick with RFT. The TPMS can be easily transferred to your new wheels. In VA, I would definitely go at least all-season... switching out every winter is a hassle, not to mention needing to store the other set of wheels. As for cost, go with replicas - they are perfectly fine in my experience and can cut final cost (incl. tires) in half. First we need to know if you must stick with RFTs.

The style 128 21" does exist for the E70 too btw... not sure about replicas for those though. In my opinion, as far as looks, anything is better than the 18s. (And don't forget to add the fender flares).

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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