Bolt ons - manifold/exhaust. Gears next - best bang for buck. But it really is down to the age old question of what you use/want the car for. Waste of money modding a 1.6 engine imo, when I say 'modding' I mean a good set of cams, headwork, ecu etc - the whole package can cost thousands and you wont notice that much of a benefit (In terms of £££ spent to bhp)