For a good rounded engine that will have a bit more punch yet still run nice and get ok fuel economy.

First, make sure it's in a good stage of tune, then probally the following steps.

1. Exaust is a good idea, a quality system with high quality free flowing mufflers will flow better and also allow more mods in the future.

2. playing with timing can be good, power is not allway the aim though, smooth running and having the engine give a bit more punch down low is, a good engine tuner with a degree wheel and some sloted timing belt pullys can really help put the power where you want it, and generaly if it's done right, you will gain some power. But be carefull, if done wrong, you can loose power and make the car less drivable.

3. cam, a mild grind cam shaft is a great way to give a real boost of power, talk to someone about a mild grind, sometimes called a stage one cam, the cam is best done with the timing stuff, and the good part is the head stays on the engine and cost is kept low.
Throttle body mods can be done in this step, but generaly it only usually gives a little power and may not be worth the cost at this stage.

With these things done you'll have a smooth grunty motor, still with good fuel economy and will hold together forever, with gains of up to 25kw or maybe 25%, it goes without saying that good quality components should be used, with new plugs leads etc if needed.

4. this is where it starts to get expensive, this step involves removing the head, a good port and polish, maybe deck the head a bit, which increases the compression and 3 angle valve seats to get better sealing, a big cam is also part of this stage.

5. Finaly here is where you start ripping off the intake and port matching and modifying the throttle body, generaly it's stuff you only do when you searching for the last few kw's of power, as it's kind of pricey for the small gains you generaly get.

After this stage you can have a engine that really pumps, dependign on the compression gain you went for and just how big you went on the porting, maybe 50kw or 50% above stock, using more fuel and putting more of a load on the engine in terms of life span.

6. Go nuts, forged pistons, gapless rings, monster lift cam, custom intake system, you name it, probally good for 100% power gains over stock, but not good for you wallet in terms of fuel bills and the need for much more regular engine servicing.

Generaly it's best to start small and just work to a stage where you like the ballance of power and enonomy.