I usually design for 1000px as well. Good luck.
The site should look similar on all browsers using css, so long as the code you are using is supported. For mobile you make a different version of the site and put some code in that checks for the mobile browser and redirects it.
Code:
<script language=javascript>
<!--
if ((navigator.userAgent.match(/iPhone/i)) || (navigator.userAgent.match(/iPod/i))) {
location.replace("http://url-to-send-them/iphone.html");
}
-->
</script>
and
Code:
<script type="text/javascript">
<!--
if (screen.width <= 699) {
document.location = "mobile.html";
}
//-->
</script>
for example.
Good luck