I'm experiencing the same problem since I switched to a Geforce GTX 460 (1GB RAM). Unfortunately I'm working on a project and was importing countless images before my old card broke. The real pain is causing the Picture Editor while trying to set an action point. When you touch an image and move on to the next, the image before gets distorted and additionally stored like that! This results in sprite animations being impossible to be set up with action points.
I've tried different driver versions of the card, all resulting in the same. Photoshop and other image manipulation programs work fine though.
Maybe somebody of the Construct dev team could have a look into that?