I think 1HP is a bit low...If you replace the air intake track up to the plenum, you'll have much better throttle response, sound, and power. Most people get the Z-tube because it's a strait pipe, and inexpensive compared to other aftermarket ones. From there, you could go a few ways.
First, if you really want a cold air intake there are a few decent ones available, but the only bad thing is that the filter is positioned in a low spot under the front bumper. A relief valve is needed in case you hit high water. (water in the intake is bad)
Second, a cheaper option is the JWT PopCharger. It will give you similar performance gains compared to the CAI, but about $100 less.
Another item you should look into is the plenum spacer. This will increase the volume inside the intake plenum to give you a boost in HP. I believe the 03's are good candidates for the MREV upgrade, which can enhance the power gains from intake mods.
Do some research, you'll find the solution that suits your needs and price limits. Here's a few websites to get you started:
http://www.g35parts.com/9083/osc/index.php
http://www.z1auto.com/index.asp