[Tristan Lall]

With any other processor family, I'd wonder if the 64-bit version was set to use two threads, and the 32-bit version four. Render operations tend to be very closely correlated with the number of cores and processor speed, and are usually pretty easy to distribute across multiple processors without significant penalties. If the 64-bit software was ignoring two cores, this would usually result in exactly that type of performance pattern—exactly twice the performance.

But this being a quad-core Core i5, you actually wouldn't expect exactly double the performance, because of the "turbo mode". On this architecture, when cores are inactive, turbo mode will automatically increase the clockspeed on the remaining cores (by substantial amounts, in some situations), in order to take advantage of the thermal headroom afforded by inactive cores. In essence, these chips overclock themselves dynamically to operate at the highest clockspeed at which they are stable and not overheating.

In 3DSMax, rendering is all about hardware floating point capability. Has nVidia come out with a driver to offload some of the rendering work to a GPU (to lessen the load on the i5's FPUs)? If you're just using a conventional video driver, it won't be doing much of anything while the processor is busy rendering, so it's probably not the problem here.

Just for comparison, try rendering one of the built-in demos on both platforms, with the default settings. (Eliminate as many variables as possible, and see what happens.)