Thank you for the info, pixelrush. So i'm going to order an EVGA GeForce GTX 580 with 3072 MB/384 Bit to be prepared for bigger projects and keep my GeForce 9800 GT/1048 as the UI display device.

Sorry, if that was mentioned before - but i'm wondering what will happen if i use a GTX 580 with 1,5 GB RAM and the textures/render size is too big. Will Octane crash or will the render be slower because of swapping to hard drive (but finally i will get my rendered picture)?