Ok so in this case mainly it's your render resolution, and second probably your GI settings (relative detail of 4 is quite high) paired with a main detail of 0.95. At that main detail level, I would either reduce the Relative Detail to say 2 (since it is "relative" detail, it will actually still be fairly high with your high primary detail), or you could reduce main detail to say 0.75, and still reduce GI relative detail to 2 or 3. That should help with memory issues.
At this point there are a few other bits of info that will paint a clearer picture of the memory situation here. First, are you on a 64 bit version of windows? If not, you'll be at a disadvantage, but you may still be able to enable the "/3gb" switch to give TG2 a bit of extra memory.
Also the makeup of your scene obviously makes a difference. Do you have a lot of large objects, populations, or large image maps (even textures on objects that are large)?
Finally, it's important to consider the atmosphere and cloud sample levels, as they contribute to both memory use and render time as well. 2 settings in particular that are often used erroneously and can contribute dramatically to both high render time and memory usage are the "Raytraced Shadows" options on the Quality tab of both cloud layers and the atmosphere shader. I recommend keeping those *unchecked* in most cases. Cloud samples can usually be set by adjusting the Quality slider and keeping it around 1 or 1.5. I seldom need to go above 2. Atmosphere samples will depend quite a lot on the lighting and perspective in your scene, but generally 32 is the highest you'll need to go unless you have heavy shadows going on.
Have you rendered this scene at lower resolution successfully? If so posting your earlier render would help give a better sense of the total scene construction.
- Oshyan