What to get for a POS

I have an automatic 97 civic LX. I plan on getting about $1,000 back for taxes and I want to put it all into the engine. But I don't know what to get. I know I will never get anywhere near what a manual could do, but it'd be nice to be a little quicker. My roommate says I should get a $500 Zex nitrous kit (which is tempting), but the nitrous would only be for racing and I'm after more hp for all the time. I'm kinda after the most hp for what money I have, so any ideas on what I should get?