The e46 screen will physically sit into the opening, but it will not integrate into the interior trim at all and will look like crap. You'll notice that it has a ridge about a third the way down the unit, which is meant to blend into the dash lines on the e46. The e53, like the e38 and e39, don't have this. If you are going to do this, do it right and get the screen that fits the X5.
Below is a picture, from front to back, of an e46 screen, an X5 16:9 screen, and an X5 4:3 screen. I had all three at the same time when I was doing the 4:3 to 16:9 conversion. Trust me, the e46 screen would not integrate very well in an X5.
