You can leave the thread settings at default or otherwise set the max threads to 4, since you have a quadcore.
If you increase the min thread to >4 threads you'll indeed create more threads, but also an overhead because you have only 4 cores.
This will slow down the render.
You mention detail settings of 6 and 7. I presume and hope you mean 0.6 and 0.7?
What are your other rendersettings, like detail, AA, GI and raytracing of atmosphere/clouds etc.?
2GB of RAM should be sufficient for an image like this, but it is not that much for heavier scenes (quality or resolution-wise).
Martin