First! Change the air filter. A K&N (Or similar) air filter element will increase power and throttle response for about £50. Better saving for a full induction kit though, replaces the air filter, airbox and induction hose. Will increase power further and make your car sound a bit sportier. An HKS one is about £350.
Next stop should be the exhaust. Go for a full system and have the catalytic converter removed. Will make it sound much better and give you a decent power increase. Prices vary but a decent stainless exhaust will cost about £3-400
Next would be a chip replacement/re-map. About £350 for a re-chip but Galant 2000 are not exactly common so you will have to look about a good bit for one!
After this modifications become a bit pricey and will affect the cars driveability. Ported and polished head, engine rebores, high stall torque converters are all well over the £2000 mark.
Try the air filter and exhaust modifications first.