Testing for issues and web compliance (e.g., W3C) compatibility in a Mac environment is not easy. It used to be that you had to both pay for licencing to one of the proprietary virtual machines (e.g., Parallels or VMWare), and also pay for Microsoft Windows licensing. I recently, however, ran into an open source alternative to these virtual machines, and it works wonderfully. It is called VirtualBox.
VirtualBox works about the same way Parellels or VMWare functions. The installation process partitions your hard drive allocating a pre-specified amount of memory (if you run Windows, specify at least 1024 MB) to the virtual processor. VirtualBox runs Windows (NT 4.0, 2000, XP, Server 2003, Vista, Windows 7), DOS/Windows
3.x, Linux (2.4 and 2.6), Solaris and OpenSolaris, and OpenBSD.
Given that IE6, IE7 an IE8 do not always run on the same Windows installation (IE6 & IE7 do not run on Windows 7 at all), you have to install multiple versions of Windows. VirtualBox can run multiple virtual machines simultaneously, so this is no issue.
To get IE7 & IE8 (and IE6 if you wish), I installed Windows XP two times. I installed two versions of XP, labelling one "Windows XP IE7" and the other "Windows XP IE8." Within each virtual machine, you download and install the corresponding browser, and presto! You can then test your web development in five or six different browsers at once. I usually test in Firefox, Safari, Chrome and Opera (Mac-based versions), but now can simultaneosly test in IE7 and IE8 within both virutal environments, all open within the same Mac environment. No need to shut down or have two machines.