I think Damo's on the right track with the gamma issue.
Sounds like you get a different gamma when Photoshop CS4 uses the GPU for on-screen manipulations, but not the right one when rendering without the GPU.
I don't know what lies in the Preferences | Performance | GPU settings area; does the Advanced button give you any sort of gamma control?
Step one might be to get the GPU rendering and the regular rendering to agree (even if both are dark), and then to hunt for the root cause (video card settings, look-up-tables, etc., for example) that would fix both.
Sorry I can't be of more help, (my video card is too old and CS4 doesn't recognize it as suitable for acceleration.)