If you want to at least try to not spend any money, you could try unticking the "Use colour skybox" and "Use deep colour skybox" options under the effects menu, at least I think that's what they are called - maybe turning all the ship/planet detail down a notch on each.
Also, if you are running Vista in Aero mode, turn that memory hogging feature off for starters - If the game runs on your card, it's a shame - and really weird that it's only the skyboxes affected
