There's always a lot of confusion on this topic of how to get more power, so I would just like to make a few statements that should get everybody pointed on the right path...hopefully somebody will chime in if I misspeak or miss something altogether, but here goes.
Number one thing to know is that these truck come detuned right off the lot. The 5.3 is rated for 315-325 hp, but you aren't going to see all of that get used. The absolute best bang for your buck, and best thing you can do to let your truck use the power it has is to get it custom tuned (as was mentioned in the previous post) and that alone will make it feel like you are getting WAY more power than you actually are. The tune will allow your motor to run as hard as it is built to.
Next: if you just want more pickup off the line, you need torque. You can accomplish this by changing your gears, changing your tires, swapping trannies, or going all the way and getting a cam/crank setup that is built for low end power. Just don't be suprised when your top end suffers dearly for this. EG: I had an old farm truck that never went over 50km/h, so I threw in a 4.56 rear end. It could pull a house off its foundation, but i'll be damned if it could get up to highway speed.
If you want more top speed you need more horsepower, or if you already have enough horsepower you need to change the driveline to get that power delivered at a higher speed (note that your acceleration will drop steeply) eg: I had an old cutlass with 2.56 gears in the rear end. it did 150 km/h in second gear...but took a solid two minutes to get up there. As far as horsepower goes you basically need to dump in more fuel. (note that your fuel mileage will generally suffer). Of course you also need a certain amount of air to be able to burn that fuel properly in order to extract as much power as possible (this will result in higher efficiency, and possible increased mileage). Putting in bigger injectors is only useful if you have a fuel system that can provide adequate flow. Using that much fuel is only great if you have pistons,rings etc. that can take all the extra heat generated. Pumping in that much fuel is only useful if your air flow can keep up (that includes flow in AND out). Safe bet is that horsepower mods = economy loss.
This all being said, if you want your truck to go faster, push the pedal down farther. If the pedal is on the floor and it still isn't enough, then you need to get more air in and out. This is the only circumstance when a cold air intake is really worthwhile. At anything less than WOT a cold air intake is essentially useless. It will make a nice sound, and possibly get you a little better response, but the actual power gain is nearly negligible. You can get to the point where you are porting heads and putting in bigger valves, and if you are then you're building yourself a pretty serious machine.
Next stop (and probably the most important one IMHO) is your exhaust. The first and foremost thing I need to say here is that straight piping your exhaust is useless unless you have an EXTREMELY powerful engine with a turbo or supercharger, and are running it balls to the walls at all times (see: racing). By removing the backpressure that an exhaust system provides, you are GREATLY reducing the efficiency of the motor at lower rpms, and needlessly burning fuel without any real gains. Most people don't really understand the true purpose of an exhaust system, and assume it is merely to redirect toxic fumes away from the passengers and drop the decibel level of the engine. While it does do both those things, it also has a much greater purpose. Somebody who wants incredible top speeds will have a much different exhaust setup than somebody who wants a ton of power at their bottom end. Changing out your cats is beneficial if you have plugged cats. Otherwise, new trucks have fairly efficient cats from the get go, and any benefit from deleting or swapping them is very minimal and probably one of the last things I would recommend changing out. If you go and buy yourself a cherry bomb and slap it in, and cut off all the pipe behind it, be ready for terrible fuel mileage and an actual drop in power. MAKING YOUR EXHAUST BIGGER DOES NOT ALWAYS EQUAL MORE POWER. Your exhaust needs to be matched up with your intake so that a) the motor is not working too hard to force out the exhaust and b) has enough velocity to maintain a proper flow in and out of the cylinders. Bigger exhaust = lower velocity. smaller exhaust = higher velocity. A high end muffler has properties that will allow the amount of backpressure to change with the speed of the motor and let it breathe properly. A cheap muffler, simply said, will not do this. the piping after the muffler is also important in that the muffler was designed to have piping come after it, and will not work as well as it can if it doesn't have this. If you are going to spend some bucks anywhere, I highly recommend spending it on your exhaust. A performance exhaust shop knows all this and more, and can help you design the perfect system for your purpose. Be aware, though that you need to plan the exhaust to match up with other mods that you are doing in order for it to work harmoniously and as efficiently as possible. Long tube headers are great, they help a ton with air flow and getting some extra power. Shortys sound nice and that's about it.
So, my final piece of advice is to save up your bucks and get all your mods done at once. This way you can buy everything so it matches up with the rest of the package, and you won't end up having to continually change out different things to get it all working the way you want. Get a tune done AFTER you've done everything else, and that way your computer will know how to use all the goodies you've installed.
That's my rant for now, hopefully it all makes sense and is correct. (I'm working nights right now, so hopefully my lack of sleep hasn't made me delusional)