The starting point to improve the performance of your engine is the air filter and the exhaust system but once you have fitted the K&N filter and Jetex exhaust where do you go next? The next modification depends on the age of your vehicle, if it is post 2000 ish then an ECU remap, which can give remarkable increases in bhp and torque, is definately a very cost effective option. C&R have been APR agents for a number of years and will be adding these type of re-maps to this site shortly.
For earlier cars or those wanting to dramatically modify their engine a more traditional route is required - fit a piper or Kent high lift camshaft which increases the amount of valve opening (lift) and increases the period the valves are open (duration/timing). The net result of this is to increase the fuel / air mixture which the pistons can draw in during the induction stroke and push out when they are exhausted.
After or whilst fitting the high lift cam the most dramatic results can be obtained by fitting a ported and polished cylinder head. These modified heads work so well because the original head fitted to your car at the factory is a compromise, the ports and combustion chambers are left in a relatively rough cast state with only rudimentary attention paid to machining out for improved gas flow. Porting and polishing accurately removes metal, sharp edges, steps and other imperfections unleashing hidden potential in your engine for relatively little cost considering the immense work involved.