Ok, I'll make this easy. No popups! A statistical disaster. Here is a nice little that sits on top of the . I only really worry about IE 6 or earlier since its the biggest culprit when it comes to messign up my designs. Below is a conditional statement (the best way to go in my opinion).
Stick this in the head (put in whatever content you wish):
<!--[if lte IE 6]>
<div id="warning">
<h4 class="red">Your Browser Is Not Supported!</h4><br />
<p>Please upgrade to <a href='http://getfirefox.com'>FireFox</a>, <a href='http://www.opera.com/download/'>Opera</a>, <a href='http://www.apple.com/safari/'>Safari</a> or <a href='http://www.microsoft.com/windows/downloads/ie/getitnow.mspx'>Internet Explorer 7 or 8</a>. Thank You! <a href="#" onClick="document.getElementById('warning').style.display = 'none';"><b>Close Window</b></a></p>
</div>
<![endif]-->
Stick this in your external styling sheet. I would not use inline styling.
#warning {position:relative; top:0px; width:100%; height:40px; background-color:#fff; margin-top:0px; padding:4px; border-bottom:solid 4px #000066}
Style it however you want - go nuts! You can get very specific with javascript to detect any browser, so if ya know how, it can add greater specificity.
This will be a pretty little box that sits on top of your content and lets your users see that their browser sucks.