A quick fix for you to do is in the wpsc themes folder of the plugin you'll find wpsc-single_product.php file.
The section you're looking is between line 139 and 145. - obviously if you've transfered that file to your own thmee folder you need to change that file as well and then flush your theme cache in wp-ecommerce in the prsentation settings section.
you just need to swap the Old Price and Price around in the brackets or change to what ever words you want.
I'm sure soemthing this simple will be included in a future update of wp-ecommerce.