Unigine Heaven is currently the most demanding OpenGL workload on Linux. Quite interestingly, the EVGA GeForce GTX 550 Ti performed very well here and was not only faster than the Radeon HD 6770, but also the Radeon HD 5830 and Radeon HD 6870! However, it looks like this might be due to a Catalyst driver regression affecting Unigine Heaven as even the GeForce GT 240 (albeit it's OpenGL 3.3 without hardware tessellation) was close to the Radeon HD 6770 performance. The GeForce GTX 550 Ti though would not be able to power a Unigine game with as advanced features and graphics as Heaven, even at 1280 x 1024. Fortunately, the premiere Unigine game, OilRush, is fairly well optimized and not nearly as demanding as Heaven.
When upping the Unigine Heaven resolution to 1600 x 1200 and 1920 x 1080, the NVIDIA GeForce GTX 550 Ti was still faster than the Radeon HD 6770 and Radeon HD 5830, but the higher-end Radeon HD 6870 managed to fair a bit better.
Those wishing to run these graphics benchmarks and compare your numbers to these numbers in this article, simply run phoronix-test-suite benchmark 1110131-AR-GEFORCEGT60. Additionally, there are more benchmarks of just the EVGA GeForce GTX 550 Ti 1GB graphics card on Linux in the 1110127-AR-NVIDIAGEF23 result file.