The image below is a perfect example of why I chose to go with a full frame camera. Even though aquariums look very well lit, they are actually extremely dark. Our eyes capture light so well, between the pupils opening up all the way (aperture) and our retinas being super sensitive to light (ISO), that don't always realize how dark a situation is.
The low-light capabilities of my 5d Mk II has proven to be pretty amazing. This photo was taken at ISO 1600. On my old Olympus E-1 and my Canon 40d, the noise would have made this capture complete rubbish. With the low pixel density of the sensor and just a little bit of noise reduction in Light Room, the image is quite sharp and brilliant. (now if only I had the motivation to edit out the glass seam in the background....)