The i3 can handle 1080p content. I have a laptop with 1.4GHz i3 and under linux (so no GPU acceleration) it can decode 1080p to 1080p TV (HDMI connection) with HT disabled. Scaling is more taxing, but if you use Windows, then the GPU drivers will accelerate things.
So if you only need to watch HD content and offer HDD storage, then i3 is enough. The only thing for which I would buy the i5 is the AES-NI support, since my HDDs are SW encrypted.