If you do go with Template Monster, let me know and I'll tell you how you can get 20% off. If you need help integrating it with osCommerce, I may be willing to help, drop me a PM sometime
Just make sure you spend more time and effort on the business itself and not all on the website, customers look at the quality of your products/service more than they do your website. I think most would pick a simple and easy to use website along with quality products rather than an excellent website and poor products.