Aerilius is correct- 64-bit computing offers no performance benefit to SketchUp or any other realtime graphics system. SketchUp performs best on a single, fast CPU core and a high-end GPU. There are no known operations in SketchUp that would benefit from a 64-bit computing environment.
3DS Max has an integrated offline renderer– which does benefit from both access to more memory (64-bit) and multiple processor cores. SketchUp does not have an integrated offline renderer.