Have you identified the case yet? Can you tell us what it is?
Another thought - where do you have the PC located? Is it stuffed into a little cubbyhole in a desk, where the hot air from the back of the PC is getting breathed back in again?
You might want to try setting it up where it's free to breathe. No obstacles in front or back. Park it about a half foot off the ground - that way it gets cooler air from low in the room, but not the worst of the dust bunnies. Run it for a bit (not Oblivion please) and use Speedfan or Everest to record some temperatures. Then open up your case, set up your room fan, and run it some more. Check your temps again. This will help you get your arms around the problem, so to speak.
At this point I agree with troppo. If you're happy with your components (not outdated) then a new case (generally speaking more cost-effective when a PSU is included) might be a fun project for you and put these temp problems behind.