First off, there really is nothing wrong with a hot attic as it is supposed to be hot for the most part.
The biggest issue is make sure you have:
- a free flow of air in the space (no power vents),
- any air gaps are sealed, and
- you have enough insulation PROPERLY installed. I would say R38 at minimum.
If your roof needs replacement soon you may want to consider going with a cool roof to help block some of the heat from getting in there in the first place. As for the room above the garage, I am not only wondering how the underside is insulated but how the room is (i.e. knee walls, cathedral ceiling, etc).
Really the only way anyone can help you is by seeing it. I would look for energy audit programs in your area. In some cases one provided by your local utility company may fit the ticket, while in others you might be best looking for a third party type by someone who understands older homes.
For more information:
Also, read this post on Sean's blog about air sealing.