Whether a frame can be processed by the CUDA hardware acceleration portion of the Mercury Playback Engine depends on the size of the frame compared to the amount of GPU memory. To be processed by the CUDA hardware acceleration, a frame requires ((widthxheight)/16,384) megabytes. If that value exceeds the available memory, Premiere Pro will use the CPU only for rendering of the current segment. This means that some images will not use CUDA acceleration on some cards but will on others. For example, one image size from a Canon T2i is 5184×3456. Doing the math, this requires 1,094MB, which just exceeds the 1GB available on the Quadro FX 3800, but is still within the 1.5GB of the Quadro FX 4800.
The point is, you'd need unusually high res clips to exceed that 2GB.