What you need to do is copy the html.tpl.php template from modules/system/html.tpl.php to your themes templates directory, so sites/all/themes/your theme/templates and open it up. In the head section add this tag:
<meta http-equiv="X-UA-Compatible" content="IE=8" >
This tells Internet Explorer to specifically use the web standards incorporated into IE 8. This means that basically for any version of Internet Explorer equal to 8 and above you'll have, at minimum, IE 8 viewing standards. I had to do this same thing when using CSS aggregation.
BTW, you don't necessarily have to use version 8 as the base, if your site runs fine with 7 then I'd recommend going with 7.