spark plugs probably aren't going to do much for you. a k and n air filter and crap like that is expensive and doesn't really do too much of anything. if you don't want to spend a lot of money drive like my grandma and like it.
If you want more hp anouther good way to go is to get a turbo kit. For the price range that you want you are looking at an extra 10 to 30 hp depending on the brand. go to this site for a few ideas http://www.horsepowerfreaks.com