Start with adding Modernizr or HTML5shiv to your site, which will enable proper display of HTML5-only tags (HEADER, FOOTER, SECTION, etc.)
Does the client insist on having rounded corners, box shading and other CSS3 fanciness, then you’ll have to either use images (lots of work) or try PIE (known to be difficult to get working sometimes). If he doesn’t care about those fancy extras, then you shouldn’t have too much to worry about.
Of course you might end up with a few IE7-specific bugs that you’ll have to fix, but if you created valid/proper HTML, then you shouldn’t run in to all too many problems.
IEtester may not give you 100% correct results, but you also have the F12 tool in IE9, where you can emulate IE7 mode.