Seeing as USB is 500ma @ 5v, the maximum load would be 2.5w... which is nothing at all (so even if the G19 took all it's power from USB - it's still inconsequential)
There are two big differences:HDMI incorporates content protection called High Definition Content Protection (HDCP). HDMI supports audio in addition to digital video. (DVI only supports digital video)
Edit* On picture wise, they are identical. You might call HDMI as ajust newer version of the DVI.