Well, I have 64-bit 3ds Max 2010 reinstalled under Win 7 Pro 64 and have been running it through some performance tests. My old comp was basically identical to my one at work and I have been using that as a baseline for comparison.
The new rig is rendering the same scenes approximately 50% faster, which ain't too shabby. I'm sure the quad-core CPU vs. the old dual-core is largely responsible for that. Generally, twice as many cores doesn't equate to twice the performance, so at least some of that must be coming from the step up to 64-bit and/or Win 7.
Viewport performance is about the same, which stands to reason since I have not yet upgraded the graphics card. I've heard horror stories about very poor viewport performance in Win 7 but that doesn't seem to be true at all in my case, even with the full Aero UI enabled.
Biggest problem I've encountered is that Distributed Bucket Rendering, which allows you to network additional computers for added rendering power on a single frame, was not working after the upgrade. My new 64-bit system was refusing to communicate with the older 32-bit systems. I finally traced this to a mismatch between the port numbers Mental Ray uses to communicate with the satellite renderers. Apparently, the 32-bit and 64-bit versions of Max default to different port numbers, but a manual change easily fixed the problem.
So far so good, and I haven't even optimized my memory settings or overclocked the CPU yet.