Your system is quite larger than mine (Win98SE, 850MHz, 512Meg) and the only issue I have had was when trying to render large hair props like RDNA's wizard hair/beard. I too tend to make complex images with several charaters, clothes, props, transparancies, etc. You might try finding if it's a particular light or maybe stage that is causing the freeze. Are you using complex Light Gels? Do you have something with massive transparancy to it? Are you using a large hair prop from the Hair Room? Does it freeze when rending a 'shadow map' (if so, which light), or 'adding props', or 'rendering scene'?