Mr Wig...
I am shopping for an intake for my '02 Navi right now and leaning towards the good ol' K & N System. They marketing stuff promotes 13.5hp gains in the 5.4 engine, but I just read something in a Truckin' magazine that says that you should shave about 50% off of whatever hp gains you read on the box.
I've used the K&N intake systems on all my rides and they help, but to be honest, I never noticed a significant difference. I'm also planning on getting a programmable performance tuner here in the next few days as well, which will actually give me 60+ hp and better gas mileage. Still researching what model to buy and I plan on spending $300 - $500 for the parts alone. After that, a new exhaust sytem.
I'll be getting one here in the next few days and will let you know how it works.