The ATI runs hot enough on its own. We're talking a noticeable increase in heat output. Even the air-intakes at the front of the machine are a touch warm, and when I open the drive-bay cover it's warm. Neither happened when the machine was just using built-in video.
On the other hand, the DVI connection provides cleaner, clearer video than the VGA connection. No more fiddling with display settings on the screen, just auto-adjust back to factory defaults and it's fabulous.