what year E do you have? This is important (fill out your profile).
Earlier E's are easier to make a custom intake for as you don't have an MAF sensor to rig. I basically just replaced the huge resonator box (left the rubber tube that connects to the TB) with a short tube (puts the filter towards the back of the engine bay to pull the air from under the car) and a filter (AEM dryflow is the best) on the end with a little heat shield (to mke sure it doesn't pull hot air from the exhaust manifold area, you can find these on ebay for cheap and it just attaches to the filter with a hose clamp). Total cost was around $75 ($45 of that was the filter). Throttle response is vastly improved. Sounds good, power gains are, well minimal, but they are on most intakes. I think it gives it a little more Pep, but its not a ton.
Hey thats my post! I saw the parts in Auto Zone recently. its a SRI though not CAI, I've found that i get better gas milage by simply removing the ressonator box and rubber tube. It works real well and cost's way less. Plus you have better protection from water.