If you want to stay away from Smog Regs then don't touch the engine. too much I think a CAI is fine, but it doesnt add signifigant horses. I don't think it will void the warrenty unless your dealer is a dick.
Do gears, and a T lock, you want amazing punch at the line go with 4.10 gears, and a t lock and possibly a
tranny cooler because it's gonna shift quick. but you wont be able to go very fast on the highway.
you can also do 3.73 gears wont get as much punch off the line but at least you be able to do highway speeds a little better.
I dunno if these other things void warrenty on the engine, but you can do Underdrive pullies or roller rockers. they will help free up some horses on the engine. I know Cali is strict. I just dunno how strict, I'm from Jersey different world lol.
But some people will give out loads of info here without a doubt, good luck with the 04
