Things to check:
Renderer -> Advanced -> Size of subdiv cache. You should need no more than 1000. Smaller values are needed if your scene is using a lot of memory elsewhere, such as in populations, objects or heightfields. A good way to be sure that you will not run out of memory for the subdiv cache part way through a render is to enable "preallocate subdiv cache", but that allocates the whole amount at the start of the render which is not always ideal.
Do you have anything reflective in the scene, e.g. water? TG2 should be able to render reflective surfaces but they can cause of problems. They will make heavy use of the subdiv cache, making it especially important not have the right settings for the subdiv cache.
Imported objects can use a lot of memory leaving less for everything else. Populations will also eat a lot of memory easily, whether or not the objects in the population use much memory.
If you can send us a .tgd file and some idea what objects and populations you're using, that will tell us a lot. (You can email us directly if you don't want to post it here.)
Matt
