Ok, so you don’t want to mess up your Mac…but if you are serious about web deign you need to be able to test on a PC platform, use an old one.
About you suggesting to your client to update to IE8, how are you going to suggest to all his clients that visit his site that they need to update too? There are still a lot of people using IE6 and 7 even if you and your client don’t, so it is a good idea to make your site as viewable as possible with these browsers even though you might think it a whim.
In IE7 you have in .main a margin-top of 110px, if you make a CC for IE7 and remove that, the main content will move up. Didn’t check in IE6.
P.S. You also have a few validation errors which could cause you some problems in IE.