well as the saying goes..."speed is a question of money, how fast do you want to go?"
the ecotec is a great engine, and anyone who says otherwise is being pig-headed (because they will most likely tell you to get a v8).
you have heaps of options with modding the ecotec and more and more performance shops are realising their potential these days.
how much money you have/want to spend will help to determine a course of action, but for most people, simply freeing up some airflow is a good starting point and the ecotec responds well to it.
fitting a 2 1/2" catback exhaust, and a cai (cold air intake) with hi-flow air filter will do this. after that its hard to recommend where to go as i dont know what you want to achieve and what kind of budget you have.
but things like high ratio roller rockers, shift kits, 3.45 diff gears, chip..etc..
are possibilities.