Not sure I got the question, however:
$.browser.msie ? $('#myDiv').css({opacity: 1}) : $('#myDiv').animate({opacity: 1}, 300);
Also, I'm not an expert of IE anymore, but I wonder if you could do something like:
$('#myDiv').animate({opacity: 1}, $.browser.msie ? 0 : 300);
That should be applied immediately, and made it more readable with an external variable:
var speed = $.browser.msie ? 0 : 300;
$('#myDiv').animate({opacity: 1}, speed);
Or something like that.