IE7 does process min-height but IE6 does not, so you may need to use a conditional comment with a fixed height for IE6.
You have a height for #container 800px which should be unnecessary as the divs inside will create its height if #header and #main have a height or min-height.
You have used height:610px three times, for #main and floated divs #nav and #body #inside #main. I changed these three to min-height: 810px and then 310px as a test and deleted height: 800px in #container and the page still looked OK but longer or shorter. So for IE6 you need a conditional comment with a safe height for all three just in case someone increases text size, and remember that different browsers use a different height for a block of text even with the same font-size due to other default sizes like character spacing.
You have this <p class="firstNavHr"></p> inside a <ul> tag which is not allowed. After deleting this or moving it outside the list, your page validates.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.