The most they can handle with all stock internals is about 250HP. From what I've read, GM tested them with a single stage shot of N20, which goes from 145HP (stock) to 250HP instantly (or as close to instantly as you can get). A bit harder on an engine that gradually building boost. But to be safe, stay at or below 250 HP. That's about what mine has, and believe me, it's plenty for such a light car
Now, another thing to consider... if you're on a tight budget, why are you puting money into a car's performance? Save up and once you can afford it, just go all out and do it right. Everything is more expensive when you have more power. You might have to buy a higher grade of gas, you'll use more gas to go the same distance, it's much easier to get speeding tickets.
If you're going to do it anyway, the "best" way will depend on what you want it for. N20 is by far the cheapest initially, but you have to refill the tank. I think they say one $60 tank lasts for 4 quarter-mile runs with a 100-shot. Gets real expensive real fast, plus you only have a mile of 250HP and after that you're back to 145HP. I'd spend $120 a day just driving to and from work
I'd go with either a super- or turbo-charger. A few months ago Sumit Racing still had some GMPP ones left, not sure now. Very easy to install, took me about 7 hours to do it... If you can read you can do it. If you go that route, add $900 to the cost... I'll explain why if you go with that supercharger. The downside to superchargers is that they heat the air more before it enters the engine, thereby limiting the horsepower that can be gained. Not an issue if you want to stick to a stock engine.
If you aren't upgrading the engine, there's no reason to go with a turbo, unless you want to eventually upgrade the engine. They're more complicated, require much more work to install, and require 30-60 seconds of idling before shuting them off.
What I meant when I said "add $900 to the cost" above, was for HP tuners... Another option is just tuning it with HP Tuners. Check out their site for accurate info, but if I remember right just by tuning an LS-1 engine properly, you can get 150 HP more out if it. They're about 2.5 times bigger than the ecotec, so 60HP might be possible from an ecotec doing nothing but tuning it. If you go this route you'll also need a wideband AFR gauge. You might want to do this first, because if you add on anything
else, you'll want to re-tune the PCM. Even changing the intake or exhaust will benefit from tuning.