First off, get some good tracking software if you don't have any. With that amount of traffic, you should be able to justify a very robust tracking system. I personally would recommend clicktracks, for a site with the amount of visitors that you have. You will be able to track your visitor's paths through your site and see exactly where they are leaving. This is the first step in increasing your customer conversion. Without any idea of where your visitors are leaving or where they're even getting to your site from, its pretty much impossible to give accurate advise on what you can do to improve. If your visitors are un-targeted, then you wouldn't expect a good conversion rate. Conversely, if every visitor goes to your site looking for something that you sell, then you should have a very good conversion rate.
As far as general usability goes, there are several things that I quickly noticed that I think could be changed to make your site more efficient.
First off, the page size is fairly high, over 100Kb, making it slow to load for people with sub-broadband internet connections. An easy way to lower this immediately would be to change the image based links in the header and footer to text links. Check your site at:
http://www.websiteoptimization.com/services/analyze/ to get some more ideas to speed it up.
Secondly the homepage doesn't make a lot of sense to me. Why are those products on there specifically? Are they the most commonly purchased, or are they on sale, etc. I think you should reserve the front page for specials, and best selling products, and label them that way. Also, making a small list of 5 of each type would make the page size smaller, and would give more options for the home page.
Next, the search and cart text boxes at the top of the page are really confusing. I cant figure out why anyone would want to enter something into a cart text box and hit the go button. I would put the search box, above the left navigation, completely remove the cart text box, and get rid of the extra left over dark blue area by moving the main content directly under the top nav links.
Next, on the shopping cart page, the 'continue shopping' and 'checkout' links are confusing. I think you would be better off with a plain text link or plain submit button. These links should be very easy to recognize.
On the orderform page:
You should still show the cart contents at the top, so the user is sure that they are ordering the right thing. The shipping prices should be displayed, under the cart contents, and any changing of the shipping options should be done here as well. I clicked on the 'click to update shipping prices' button and got a dead page. That is an instant cart killer. Use javascript to validate the shipping zip field is filled, so the user cant get to the dead page. Again, the continue button should be easier to recognize. Also, if you don't need the fax number of your customers remove the field from the order form.
Anyway, this is what I came up with from a basic look at your site. I think that if your visitors are even moderately targeted, you can see a huge increase in sales with some good usability modification and some good analytics.
I highly recommend that you read these books and put their recommendations to use on your site, especially the first one:
Steve Krug - Don't Make Me Think
37signals - Defensive design for the web