Last week I also had a project with large stills (4000 x 3000 PNG) and I also found the machine could not handle the work well so I disabled MPE and the machine worked better under those conditions without the help of GPU. I had to zoom in on the product "Sushi" and move around the plate and then zoom out.
try disable mpe and then render.
How much video memory is on your nVidia card? This could be an issue where the GPU memory is getting overwhelmed by the number of pixels being pushed around at any one time.
To piggyback on what Jeff mentions, here's a post from Todd Kopriva (Adobe) about maximum image sizes and CUDA: maximum dimensions in Adobe Premiere Pro CS5 and CS5.5 « Premiere Pro work area
The gist is that Premiere Pro in software mode is able to handle much larger image sizes than Premiere Pro in hardware-accelerated mode, since you're limited to the RAM available on the GPU. If you want to use GPU-accelerated rendering, you may have no choice but to decrease the size of your images slightly--or invest in a card with more RAM