Motherboard with DVI, VGA and HDMI : How many can I use at once?

"I've got a Gigabyte motherboard with DVI, D-SUB and HDMI onboard and I know that doesn't allow use of HDMI and DVI at the same time.

From looking at the manual for your motherboard (which is available from the QVL page at http://www.asus.com/Product.aspx?P_ID=EbXqq06yxpJwlABf ), the same is the case for your motherboard. You can use DVI and D-SUB at the same time, or D-SUB and HDMI at the same time, but not DVI and HDMI.

DVI and HDMI are variants of the same standards, or HDMI is a superset of DVI. Unlike DisplayPort they also require a separate clock per each connection, meaning there is a hard limit to amount of DVI/HDMI connections you can use at once. On most discrete cards the limit is 2, on integrated graphics it varies."

it depends on the number of provided sync clocks. Most common is 2 out of all provided connectors, one digital and one analog. But there are also solutions out there where either only one output at once is supported or two out of all available.