What is commonly known as "Stage 1" (exhaust, de-cat, chip, filter/induction kit) will probably net you about 5-10bhp in the real world, make the car sound great and iron out any flat spots. This lot will cost under £500 all in, but wont make your car much faster and the 16V is a very expensive car to tune beyond this. Cams/head porting etc will cost around £2000+.
Probably the cheapest way to get guaranteed extra power and torque is to hunt down a 2.0 Williams/Megane Coupe bottom end and slap in onto the Clio 16V. Not a lot more power, but the torque advantage is massive. Thats what a fair few Valver owners have done and the results are cars that 1/4 mile faster than most 172s, Cups and even Escort Cosworths (Adis tuned Mk1 16V 2.0 beast). In fact, Im doing that right now!