Help with conditional stylesheet

Hi all

I have put instructions in my header.php
<!–[if IE]> <style type=”text/css”>@import “IE-override.css”;</style> <![endif]–> if using interet explorer. Basically the header margin needs to be reduced by -20px as there’s a White gap above it.

My question is how do I write the actual css file that tells ie to reduce margin by -20px?

Thanks in advance

tim