Pgarner's right. Generally the more BHP you want the deeper your pockets have to be.
Without going over the top you could fit a cold air induction kit. Fit a De-cat and an aftermarket branched manifold with a full exhaust. Replace the plugs and leads with aftermarket performance one's and also get it chipped. But for the money you spend don't expect alot of bhp gains. Maybe 30-40bhp increase on the mods listed above tops.
Next gets serious money cams, pulleys, porting and polishing the head, gas flowing, getting the crank balanced and knife edged. con rods, pistons supercharger. But thats gonna cost.