Hi am also facing the same issue with my renders.
Let me give you all the details, so that you can suggest me the upgrade.
System Config :
OS : Windows 7 - 64 bit
RAM: 16 GB DDR 3
Processor : Intel i7 2600K 3.40GHz
Graphics card : NVIDIA GT 610
Software : 3ds max 2014, Vray 3.0
Render settings :
https://youtu.be/0y6byA-oePI
Lighting
I have used Vray sun, Vray Dome and some Vray sphere with a general subdivision of 30.
Also am getting noise in the trees while rendering a walkthrough. Sample of the video has been given above in that video link. Please refer it.
Since because my scene is having a quite large no. of trees, the file size is 3 GB.
Please tell me what should i do to increase the render time and is there any way to reduce the file size ?