I'm finally buying myself a 5m long HMDI cable so that I can finally hook up my computer to my 42 inch HDTV without having to move the system itself. I'm pretty happy with this, because it means that I can show PC games to my friends without having to awkwardly crowd around the PC monitor. However, I just have two questions in relation to this.

1. Can my PC output to both the monitor and the TV at the same time?

2. What kind of stress does this cause on the video card? Would it render everything twice at different resolutions or is it more efficient than that? (I have a GTX 650ti)

Edit: I don't have the cable yet, so that's why I'm not just experimenting to find out.

1. Windows natively supports two monitors, so no special software is required. Just plug that extra cable into your GPU and set up the second monitor by pressing the Windows key and "P" until you've selected the extend option.
2. Since you won't be doing eyefinity or whatnot this really won't cause any stress to the point of performance loss gaming wise.

It's not much stress, your PC should be able to handle it fine if it does your monitor at 1920x1080 already.

Now that you're going to be using dual screen, and I assume not 100% of the time, here is your lesson. Windows Key + P. IN Windows 7 and Windows 8 this brings up the menu for selecting your display devices. Some catches on it though. If your monitor is DVI or VGA but your television is HDMI, it's going to call your television the "PC Screen." Your monitor will be the "Second Screen." The four options are: PC screen only, Duplicate (mirrors the images), Extend (you can manage apps on both screens separately), and Second screen only. If you get into 3-screen territory like myself it gets even weirder. If I want to use my 2 27" monitors I tell Windows to do second screen only. If I want all 3, I do extend.

Also, when you set up your dual monitors be sure to go into the display properties and set your main display. Get to it by right clicking on the desktop and picking Screen Resolution. From there you can set your main desktop screen. I'd recommend keeping it as your monitor and using the television as the extension because of weirdness with desktop icons.

@toxeia: Cheers, thanks a lot for that. I've never used a mutli-display set up before, so this crash course was very helpful.