93 octane tune on my 2000 Tahoe with 200,000 miles. World of difference. Then it got totaled so I'm going to take my 2003 to Justin next month for my 2nd BB Tune. Worth every penny.
Before you look at mods, make sure your engine is completely tuned up. You'd be surprised how much power you will lose over time. Make sure you replace your plugs, wires, fuel filter, clean your MAF and throttle body, do a top end clean with Seafoam, and make sure your battery is good. These LS engines are just as much computer as they are motor. A weak battery or bad cable will cause problems especially when you're engine is under load. On the other hand, a good tune will make your other mods more effective.
Anything beyond tune-up, CAI, Exhaust, and a BB Tune you are going to have to spend some money. Cams, headers, heads, superchargers and such make much more power but cost an arm and a leg. Around 350 hp is your limit without dropping some heavy dough.
Someone correct me if I'm wrong since I would love more power for cheap if it exists.