Sounds like you have standard plugs for that cost. The correct ones are iridium or platinum cant remember which! Silver box from NGK rather than Yellow anyway.
order some Iridium Power .......i cannot stress how much better my car runs over the standard NGK plugs. I used to have an intermittant slight misfire on load @2k rpm, but thats gone, these plugs only thing thats cured it.
I used to use NGK before, but i wont be going back.
http://www.k-tecracing.com/show_product.asp?id=2541&appid=2