How many threads are there on this subject already?
Throttle body injection won't yield more horsepower if your engine is tuned correctly as it sits. If it runs like crap you'll notice a difference.
Multi-port injection will yield horsepower gains, though they won't be huge.
In the end it comes down to drivability, reliability, and if those two items are worth it to you to either spend the money for a kit or invest the time it takes to figure out how to do it yourself and save some cash. It still isn't cheap to do it yourself, but you can save a significant amount and learn something in the process.
Lots of info has already been posted here and on the rest of the web.