You must also rememebr that, if you provide enough thrust, you can theoretically move at the speed of light. Remeber in space, there is nothing natural to stop you, other than the obviouses.
Why we sometimes see cap ships move as fast, or faster, than fighters is explained in just about every book. They close the scoops so that there is no drag, and cap ships carry enough fuel to move at rediculous speeds. There reason why they don't seem to move in combat is that it would be impractical to fly at those speeds. It would also be impractable for fighters. While I'll let LOAF provide the full detailed answer to your question (as he proclaimed), your questions can be answered with a little knowledge space flight physics.