also: try using mentalray (it's a bit better at memory management) or there's a checkbox in the render setup window in the "renderer" tab down the bottom that reads "conserve memory" which makes things run a bit slower but may stop an OOM crash
Also uhm ... you might be disappointed when using MentalRay with a shader like 3Point: it comes out just flat grey. Use 3Point ViewThief or GrabViewport 2.5: you can "render" high res, shaders will look the same and you won't run out of memory.