I have spent many time reading Jalopnik and Google+, I have found this advice from many of the people there. I can not claim the veracity of this advice, only that it will guarantee minimum 100hp increases.
Check your brakes! Are they blue? Do they have rust on them? Are they not F1 track ready? You should replace your brakes even if you answered no to all but the last of those questions. You need racing breaks made of Carbon paper so you can brake late and be fastest to the Stop n Shop. Brakes save precious tenths on teh race track of life.
Washer fluid. Do you have some? Drain it, it weighs too much.
Is your car interior brand new? Does the interior of your car look like this? Still too much stuff. Remove the door things, the middle bit, the cup holders and the arm rests so you can go faster and be king of the culdesac. Horns too, people will know you're coming when they hear your enging instead of the sound of the sound dampening things.
Supercharge it. Does your car not have a supercharger? Add one, its super easy, like the charge. It makes pretty much everything faster like in NASCAR. All the best cars have a supercharger. Camaros, Firebirds, Honda CRX, Smart. You cant deny the results and its supereasy to install just put it on top of the engine and bolt it on their.
Turbocharge it. Do you have a supercharger? Do you not have a supercharger? You should add a turbo, its pretty easy you just drop it in and screw it down. You get many more horsepowers and more MPG because of how the air works and the whistle sound when you stop.
OTHER THINGS YOU CAN DO
Swap the engine. Not very hard especially for an SVX or a Legacy GT. SVX's take different engines like your mom takes an unfamiliar penis. Easily.
Check the VIN. That sweet car you are buying may not be that sweet. It may be an SVX.
Swap the transmission. Just like swapping an engine except its so your Diesel Wagon Volvo can be con manuel. Drop the old one, line up the new one, push.
Spend all your money fixing it. Pretty straight forward. Buy any car that may have been fun to drive 30 years ago, dump life savings into fixing the oil pan.
EDIT Thanks to For Sweden
Turn FWD to RWD. Its pretty basic, most autoshops call it a tire rotation so just ask for that.