I think VirtualBox might be too troublesome and slow to use. My company is willing to buy a copy of Virtual machine for me I think, is VM Ware, Parallels, or other products (for Mac) the best in your opinion?
(also it has to work with the .vhd on http://www.microsoft.com/downloads/en/details.aspx?FamilyID=21eabb90-958f-4b64-b5f1-73d0a413c8ef&displaylang=en so IE 7 and 8 can be installed (to test our websites))