While Google’s Page Rank indicates how important your site is compared to other sites, your Alexa rank shows how many site visitors you get compared to other sites.
Ideally you want to aim for the highest possible Google Page rank and the lowest possible Alexa rank. The best possible Alexa rank is 1, the worst is a billion. The best Google Page Rank is 10, the worst ‘n/a’.
Here’s a handy little tool that tells you your Alexa rank. Keep an eye on the numbers and see if over time, via DIY SEO, you can lower it as your site visitor numbers increase.