Gain more performance from a Ford Focus?

I just bought a 2000 Ford Focus SE, and I was wondering what all modifications I could do to gain a little bit more performance?

It's running the 2.0 16v DOHC engine, which I know is normally a decently powerful engine, but it seems to have less power then my old 94 Nissan Sentra that had a 1.6 DOCH in it.

I was told by a friend who is a Ford Tech at a garage that the computers in Focus's are designed for less power because it gives better fuel economy. But I would rather have a bit more power and sacrifice some fuel mileage. So far, I've only given it a tune-up, New plugs (Bosch Plat 4's Supposedly the guy at the autostore said they will give a little bit more combustion which will raise performance ever so slightly. If anyone knows better plugs lemme know.) New wires, (Don't remember the name, but they were 8.5mm.), new fuel filter, and new oil and oil filter.

I haven't messed with the intake yet, because I know sometimes getting a better intake would increase performance. But I'm not sure what to look for, so any suggestions would be appreciated.

I was told about getting a "Cat-Back" exhaust and a header for it. I did some looking around and saw some catback systems for like $600… Which is a little bit more then I was expecting, but I might go that route if I get some performance out of it.

Other then that, I don't really know what else I could do. I was reading about chips that change the computer in it and boost performance but I couldn't find anything for under a grand which I was hoping I wouldn't have to pay that much. Lol.

In short if you don't want to read anything,

Suggestions on Air Intake/Filter,
Exhaust?
Is there better plugs then Bosch Plat 4s?
Any other minor things I can do for performance boost?
Possibly a "computer chip" thing?

If anyone has any suggestions, or links to sites that offer the parts you are suggesting, it would be greatly appreciated.

Platinum plugs don't increase performance at all. They just offer really long plug life. Even though they have 4 ground electrodes only 1 spark is ever made to one of them when it fires so it's not like you're getting any extra spark or anything there. They're just expensive. That's why the auto parts guy who doesn't know anything about how cars work or what makes parts better then others sold them to you.
Iridium spark plugs are the only real improvement over standard copper core plugs performance wise. Iridium has an extremely high melting point and is the most corrosion-resistant metal known to man. It's also rarer than gold and is extremely hard and dense which is why the plugs are so expensive. Anyway, because of it's robustness the center electrode can be extremely thin which makes the ignitability of the plug very low. This maximizes performance and minimizes voltage requirements. Since combustion is quite good when iridium spark plugs are used, accidental firing and misfiring is reduced resulting in a smoother running engine and improved fuel efficiency.
The first thing you want to get is a KandN high flow air filter and not a CAI(cold air intake). CAI kits are for the most part gimmicks and a complete waste of money and do nothing more for performance than a KandN for your stock air box. They actually pull in hot air 99% of the time(bad for performance) and are often made with metal components(plastic is better) as well which further increases heat to the incoming air charge. There's nothing "cold" about them at all. A KandN filter for your stock air box will give you a cheap and easy 1 or 2 hp.
Your exhaust should be your second mod. A header to replace the stock manifold will make a good improvement in power and a mid pipe with high flow cat and an aftermarket exhaust will make your car breath easier. You could get 10-20 hp or so depending on what you buy. JBA Cat4ward headers are the best you'll find for your car. $479 on Summit Racing and they're stainless steel and ceramic coated- the best of both worlds. It's a bolt on product so you don't even have to change anything else to put it in. You can do the cat pipe and the rest of the exhaust later if you want. A Magnaflow cat pipe is about $350-$399 depending on your trans. A Magnaflow cat back exhaust is about $275-$300.
Third would be a computer tune(computer programmer). After you do a couple of mods an SCT tuner is a good investment. Getting a tune for your computer will optimize your mods and you can set the computer more for performance than mileage. I don't know where you were looking for tuners or chips(chips were more used in the old days before tunable computers were installed in vehicles) but I looked on Summit Racing and saw an SCT tuner for $379 or a Steeda(also using SCT tech) for $399 for your car. Always buy an SCT as almost every custom dyno tune shop will use SCT and most people in general also use SCT so you can get tips and help easily.