a) Including the product picture again in the "Cart Contents" overlay provides better process continuity, and that usually results in better conversion.
b) Push all of your fields to the left side of the page in "Delivery Info" and "Billing Info" - that will put those fields directly into the hot-zone of attention where users spend the most time looking - the less you make your users work the higher your conversion rate.
c) The page width is a little off in IE8 and FF creating a horizontal scroll bar in 1024x768 at the bottom of the page, this is one of those tiny little professionalism issues that people can subconsciously pick up on and ruins a sale - even if you're only seeing <10% of visitors in that resolution.
d) Under payment method you have a floating radio button but no options - I'd remove the radio button, and position the VISA / MC / etc logos into the cart box.
e) Showing an SSL certificate and reinforcing security heavily around where users are actually entering credit card info can result in huge gains - more on that here http://conversionvoodoo.com/blog/2010/07/proper-placement-of...
Good luck!