Follow the HTML, XML, XHTML, CSS standards and you will be fine. A few months ago I created a quite large web site - that was on IE6. Now I have migrated to IE7 and everything works fine and looks as in IE6. No problems at all. I did not need to touch
one line of code.
Neither IE6 or IE7 follow standards, if your a web designer you should know this...IE7 is an improvement, but still a pain in the (I need to watch my language) as developers now need to cater for IE7 as well as IE6 and the rest.