The ultimate factor is, how important is what's on your computer, and how much damage can it do you if in the wrong hands? If the answer is none, then relying on Win updates and your router is enough. If the answer is "a lot", then you need to consider additional steps. In some cases, a hardware firewall doubled with a software firewall may seem overkill, but it wouldn't hurt, if you know what you're doing.

A lot of it depends on how much memory you have to spread among processes too. If all the security measures bog down your performance to a crawl, then you have a problem. But if your machine can handle it in stride, then do what you feel is necessary.