Have been asking this question myself recently. I got the 1.8 HLX and NOS seems to be the way to go if your looking for a big improvement for less than £1000. The kind of systems I have seen are between £400 and £700 for a 75-100bhp gain.
You do have the high running cost of refilling the bottles to consider as well.
There is the whole illegal side of it to think about though. There are a few people on the forum here that have done it. Ribbz for one.
Safest option seems to be Wet NOS. You can get systems that fit in to your air filter box if you have an induction kit. Just leaves you needing to hide the bottle
probably cause only people who seriously race their cars use NOS. As i understand it, you can get good boost and stuff, but not or long. If i were to spend a grand, i'd want a permanent increase in bhp. I'd quite like to take my 1.8 bravo above the 130bhp mark, but since i'm not willing to put cams and stuff in, i dont think that's gonna happen. Maybe i'll get a jtd, but its more likely that this'll be my last bravo. I'm not looking to change my car hlx until the summer of 2006 though.