2.68 Cycles GPU CUDA out of memory error though peak memory used is only ~626MB

I’m attempting to render an ants eye view of patch of ground with some pebbles and grass. Most of the grass is merely an imaged mapped to planes. The modeled grass uses three 1024px textures, the pebbles use three 512px textures, the ground itself uses one 4096px texture.
Cycles begins rendering but after one or two tiles it suddenly gives a “CUDA out of memory” error and stops rendering. Peak memory only reaches 626MBs and based on my setup (shown below) it doesn’t seem that should be enough to crash Cycles. So is there something else going on? Or do I need to just lower the resolution on all my textures until Cycles stops complaining? What am I doing wrong and how can I fix it?

My setup:
Mac 10.8.3
3.49GHz Intel Core i7
GeForce GTX 570 1280MB x 2

Thank you for your help,

Hi, the easiest way is to change the 4096 texture to a dummy texture or use on of the 512 textures for testing.
You can also try to render on the card that not drive the display, save a few 100´s MB VRam.

Cheers, mib.