Yes the keywords are very important for not only the page optimization but also with how you brand your site online and even to the extent of what anchor keyword text is being used to link to your site.

You are sort of right when you talk about using keywords in your text where you need to keep a good density so it does not start to be flagged as spam..

However if you really want to increase your web sales with keywords then you need to look at what links are coming into your website and what keywords are being used in those links. That is the key to getting good listing for your website. You want to have a look at this post How To Increase Website Traffic