If you visit TheCounter.com and look at the monthly reports for Resolution (screen size) you will notice that 800 x 600 is around 45% of all users. 1024x768 is around 40%.
Now this is not for everyone on the internet but it is a fair representation of what most people use.
Takeing this into mind your page width should be no more then 760 pixels wide unless you have your page width set to 100% so it will streach to fit any screen.
Just remember that it has to look good in the 800x600 screen since almost half of all users view your site in that way.
You should also make sure your thumbnails and main product images are not too big, or it will force a lot of your users to have to scroll to the right to see the whole page. Looks is everything and this would not look good to some people.
Also last month almost 93% of all users are using the Microsoft Internet Explorer to view pages with. So build for this browser first then for the rest.
Just wanted to pass this along just in case you needed to know.