Your alexa may improve depending on the type of traffic and how it is generated in terms of google rank it will not improve in my opinion.
Your alexa may improve but backlinks will only increase your PR and SERPS + SEO of course So if anything, when buying traffic your alexa may increase, anyway, why worry what alexa has to say, the most important thing is traffic on your site unless your planning to drive down the alexa rank to flip the site for a profit??