Simply Accounting needs a PC from what I can see on the Sage site. So you can't move to a mac unless you want to get into the mess of choosing a new accountancy package and migration, or configuring virtual machines, getting a windows license for those virtual machines and putting your accountancy software in there.

As for the Win7 versus Win8, I'll be honest, I don't think it matters. Do you want your start menu in the bottom left hand corner behind a button, or in your face when you login. Either OS will support networking (via homegroups is simplest). Facebook just needs a browser, your email won't change, etc.