No problem. I said to fix it this way because you said you were new to this.
But if you wanted to fix it the correct way you edit the file at /wp-content/plugins/wp-e-commerce/wpsc-theme/wpsc-default.css
Find #single_product_page_container { and change that number to 580.
But if you leave as it will still work. Just using !important should be avoided if possible.