A good start would be to upgrade your intake and your exhaust system. An intake can be purchased for less than $100 usually. IMO--as some may disagree--you can get a cheap intake pipe and put a good filter on it and save yourself some money.

Upgrading the exhaust can help, too. Your best bet to do this on a budget is to find a muffler that will fit on your car and bring it to a muffler shop and have them custom bend piping and weld it all for you. It will cost less than purchasing a complete cat-back system.

If you wanted to get a little more from your car other than bolt-ons, you could always look into replacing the cams or doing some head work.