A:AnswerI am using this with a MacBook, so I am not sure about an iMac, but I can close my MacBook, shutting off its screen and still use the monitor attached to the adapter.
A:AnswerThis adapter is not compatible with the old RGB. This is because the the female end is an HDMI connection. However if you meant DB15 connection this answer might help. DB15 is also called VGA. it's a 15pin connection and the HDMI can be a 4 or 19 pin connection depending what you are using it for. There is an adapter for a VGA to HDMI on Best Buys website. Do a search on Best Buys website were you can find it.
Here's a pic of a VGA to HDMI adapter. Hope it helps.