At the very least you'll need a video card that can output multiple signals, with at least one of them being over HDMI of course. Any of the Radeon cards from the 5000 series upwards will do the job fine.
Next, you need to look at cabling. Assuming that the computer that is outputting the signals is within reach of your office monitor you need to determine the distance between your computer and your television.
Now this next part is optional but you should consider how you'll manage content across the two displays. If your computer OS is something like Vista/7 then the built-in handling of multiple displays might be enough for you. Or you can look at something a little more robust such as Ultramon: http://www.realtimesoft.com/ultramon/