Software makes a world of difference for the 323i.
Not so much for 330's. But for 323s, night & day.
There's a bunch out there.
I'm running Dsylvatech software with a Gruppe M cold air intake. Shark Injector software is pretty good as well. As is Dinan but a little pricey.
For 323's... you're lookin at about 10-15hp gain. Cold air... a few horsies if any. But it helps. I can suggest going for a used CAI, as the full price is not worth the bang for buck. A used CAI will shoot you around 150-250USD depending what you go for.
I've done extensive research for more gain in E46's... the synopsis is, if you want a faster car, buy a faster car like an M3 or M5. The amount of money you spend to get gains is not worth it. I've looked from all solutions from cams/headers/throttle bodys, cold air, short ram (illegal in Ontario), software, NOS (illegal in Ontario) and blowers.
Then there's always the supercharger.