Remember, it's not so much the HP that you want to boost, but the torque, that's what will ultimately move your big beast. That being said, you need to look at things in a certain order if you want to make power without grenading your powerplant. You can easily get 100hp, but you'll add 250-300ft-lb of torque.
Start with the intake, get a cold air intake and a high ram elbow to replace the pinched square OEM one; then get a turbo back exhaust, preferably between 4-5"; a good tuner, I'm a fan of the Smarty Jr or the Edge w/ attitude, but the bully dog and banks are also great selections for the 24v. Those are the absolute basics. After that, upgrade the injectors and either add a second CP3 or upgrade to a high flow pump. You need to overfuel before you can add more air or you'll lean out and blow the pistons. You also need to overfuel to keep the EGTs down or you'll melt the turbo. You will have issues with high EGTs once you start running the larger injectors, so you'll want to add either a propane injection system (can you say no to 35MPG?) and/or a water/methanol injection system to cool the intake stream.Once you have done these, THEN look at upgrading the turbo to either a large single or a twin/sequential, but the turbo should be the last thing to look at when it comes to upgrading a diesel, you MUST have the supporting mods in place first. A fuel/air separation system is also a wise investment to keep air pockets out of the fuel delivery system since an injector dumping an air pocket is a great way to wreck the injector.
Along with the basic performance upgrades mentioned above, you really want to upgrade the drive train to handle the added power. With the basic 3 you can easily be making 700ft-lb of torque and the 68RFE tranny doesn't like to handle more than 600ft-lb, so you'll want to upgrade the valve body, a billet flex plate, and a higher stall converter (2800-3200). This will give you some reliability, but you'll start to slip once you start making the bigger numbers, so you'll eventually want to get into a triple or 5 clutch kit. At this point a billet input and output shaft would be a good ideas as well. You'll also want to upgrade the tranny cooler and add a bunch of guages to keep track of everything so you'll know if you're starting to get into dangerous territory.
Guages to look at:
Look at the mileage you have on the motor, if you're 250K+ or near that, you might want to look at getting the block resleeved before you start doing any major mods to it. Also, have someone take a look at the frost plugs and check their condition, they are notorious for blowing out and turning your lovely motor into a lawn ornament.