As a 32 bit application TG2 cannot actually use more than 2GB of memory anyway. However if you do need to adjust memory use there is one option. Although you cannot set an overall maximum for memory use in TG2, you can at least affect a large part of the memory use by setting the "Size of Subdiv cache" setting in the Advanced tab of your renderer node. The cache size is a total which will be divided among the number of render threads in use. There is a minimum size of 50MB per thread, so if you have 2 threads, the cache will need to be 100MB or larger. Smaller caches may result in sub-optimal performance, especially on complex scenes. However if you have memory constraints, I suggest limiting threads to 2 at most, and using perhaps 100MB *per thread* cache size. Setting the cache size will not limit overall memory use, however
This brings us to the second part of your question which is that your CPU does not seem to be fully utilized. It is important at this point to know the full specifications of your computer, especially your CPU and operating system, and also to report how many "Processor Cores" TG2 finds when it starts up. This will be shown in the Splash Screen. It is possible either that A: it is not detecting all the CPU's/cores you have and thus not utilizing them all or B: you are using a high-end system with a large number of cores and running into efficiency limitations which TG2 has in utilizing more than 4 cores. Your specific system configuration will have a large effect on how you need to proceed.
- Oshyan