It all falls down to your budget.
For a noticeable gain, and a generally well known good setup is as follows:
Panel filter (the ph1 172 airbox is a good design)
Cat back exhaust (You can remove the cat, but the ph1 cat does flow pretty well - if you rempve it it will be a lot louder)
Matched inlets (will fetch you 2-8ish bhp depending on how bad yours are now)
Custom remap
All this will set you back around £1000-£1200, and 'should' give you a genuine 180-185bhp.
Another thing to check is your cambelt. Check where it was changed, as unless it was done by a specialist, there is a good chance the timing may be out (You need specific locking tools to do the timing, most garages do not have these, and even renault can get it wrong) and if it is out, you will be down to 145-150bhp - a coorectly timed healthy standard 172 will have around 165-168bhp (the 172 is in PS,not bhp) so your increase to 180-185 would make a noticable (if not massive) increase, and will improve the way the car drives and will make it's power delivery smoother.
Next stage after that is 3 ways:
1. itb's
2. turbo/supercharger
3. JMS inlet manifold.
If you want more power than the level offered by the stage i listed above then leave the matched inlets etc and go straight to JMS - their inlet manifold conversion will give you 190bhp + for an outlay of £1800 odd and retains a lot of standard items - and very easy to return to standard - you can sell the kit on.
Next stage is itb's - these are around £2500 fitted, and will take power to around 200bhp and makes a great difference (talking from experience)
If you need more power then it's forced induction time - be prepared to spend big bucks though - you will need circa £3500+ to take you to around 230bhp, with scope for much bigger power.
Hope that helps
