You can't raise performance and gas mileage at the same time in 99% of the cases. As Vi_edit mentioned you can't ever make it fast. Well he's not entirely correct. Drop $15k into it and maybe you can get a little more speed out of it.
Fact is there is almost nothing a person can do to waste more money than upgrading the performance of their car, especially when it's dog slow to begin with. Cost vs. gains is just ridiculous. Not only will you end up spending 2k and still be pretty damn slow (although you may sound fast), but your resale value on an otherwise solid reliable vehicle will plummet as well.
My advice is keep driving it because it's cheap. It's tragic to see somebody with a slow car throw thousands into it and it's still spanked by a stock reasonably fast car anyway. Save up that money to put into a car that's fast to begin with - factories make performance a hell of a lot more cheaply than somebody taking a slow car and making it faster. Save your money. Save your money. Save it. When you're older you'll be glad you didn't throw money into a 7 year old car.
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