If you have a Sohc, you can replace the muffler (even with a junkyard Dohc $5 one). You can add a CAI and top it off with a MP or AFX controller. That will get you about as much power as you are going to get cheaply. It starts getting expensive after that for not that much gains.
Or, save your money for a while and go Turbo. Interesting fact I ran across a while ago, if you swap a Dohc head onto a Sohc engine, you will reduce the compression from 9.8 to 9.2, perfect for a turbo. Anyone know of someone whom has tried this poor mans method of reducting compression?