All of the templates has been tested to be compatible with all browsers, except a few of the old ones that doesn't open up well in IE6 only. These templates have been tested best with Firefox. In some templates, there are glitches when you open the Layout page inside IE7, but the actual blog opens up okay. My recommendation is to use Firefox, or download it for free if you don't have one.
Another problem you might see is that your sidebars are pushed below the Post column. It might be because some elements in your sidebar is wider than the sidebar itself (you won't see this problem if you use Firefox).
Downloading Firefox is extremely easy and takes less than 5 minutes, 10 minutes at worst. It's small in size, faster page-loading time, less crowded on the toolbars, and you can change the theme/appearance anyway you want for free.