Well graphiccard needs extra power agree,but its dedicated,it reduces load on CPU,Ram too.So you get better performance.
What performance, spirovious? All the HTPC needs to do is to play video. And for that onboard graphics card such as that found on the said Gigabyte board are more than enough. In fact I have found they are good enough to handle some games that are light on graphics!
A HTPC does not really benefit from a dedicated graphics card. Plus it induces more flux in the form of heat and noise. So, I would definitely not recommend this option.