If I understand correctly, the PC upstairs has some video files you want to play. You'd plug your laptop in to your TV downstairs via HDMI or VGA I presume, and use a keyboard/mouse to watch those files via VLC, correct? That's far from ideal, but it would work.
A better solution, proposed many times already, would be to use a Roku, WD TV Live, a cheap android box with XBMC or a WMC extender instead of your laptop, and use a regular IR remote to watch whatever you want. That way, you can use your laptop for other things and operate the TV like you would if you had a disc player or cable box. There are many other similar solutions that use different hardware and software (Plex, MB3, etc.), but that's the basic idea. You don't even need a PC upstairs. All you need is a NAS.
I'm all for running as much Cat5/6 as you can afford. But what you have already will work fine. I wouldn't recommend using a laptop as a client simply because it's unnecessarily complex and awkward to operate.