Core clock is largely irrelevant unless you're talking about 30% differences and all the other hardware is the same (shaders, bus width, memory clock, etc.).

Looking at those two cards:
Memory bus width: GTX550 = 192bit; 5770 = 128bit
Memory clock speed: GTX550 = 4.3GHz; 5770 = 4.8GHz

Looking at the charts, I'm seeing the 550Ti about 10% FPS higher than the 5770 averaged across all games. I couldn't find any specific reviews around Rift performance, though either of those should be good to go. Just set your two alts to low-quality renderer, set the background CPU options and the main should run fine at higher settings.