Cheap HP upgrades usually yield tiny little increases in HP. Aside from what dug said above (and those are all great suggestions that will yield a noticeable increase in power) you could do plugs/wires/cap/rotor/ignition to get a few more horsies under the hood.
I'd tell you about my truck but you probably wouldn't believe me.
Which header is prefered, I heard the Downey 4-1 is ideal yet others say LC Engineering or something. I have heard that some headers really aren't very good due to poor design so I want my $200-$300 to be well spent.