"A Slower Speed of Light" is a relativistic effects simulator with atmosphere, courtesy of MIT.
http://gamelab.mit.edu/games/a-slower-speed-of-light/
The hardware requirements aren't as bad at lower resolutions. My AMD A2 workstation can handle 800x600 without issues (though it chokes on the default resolution).
Edit: actually 1280x1024 (the maximum for the workstation's monitor) also works okay. I think the hardware requirements are vastly overstated. (Especially as the workstation has an onboard GPU, not dedicated.)