Why wouldn't the OEM boards fit the 4.6is?
Only thing you need to do is swap your rear flares over to it, SlickGT1 made a thread about it.
http://www.xoutpost.com/bmw-sav-foru...etter-diy.html
OEM boards run you $750, quality aftermarket boards run you approx $500-520 depending on which ebay seller you go through. Either way its a hell of a lot of coin to drop so I haven't gotten either one yet.
OE boards:
BMW x5 Running Boards x5 Step E53 Up to 06 4 4 4 6 4 8 | eBay
Aftermarket (although no one has posted yet that they have bought these and reported on quality, etc):
For 00 06 BMW E53 x5 3 0i 4 4i Running Board Side Steps Bars SUV 4WD V8 AWD OE | eBay
I'm sure if more people installed the aftermarket boards on here they would be more popular but its a hell of an experiment to drop $500 on.