Couple things worth noting for people planning on running 64-bit Windows:
1. Applications that are written in a 32-bit code base that are 64-bit compatible will not use more than 4GB of memory for their OS-scheduled process. A lot of the apps we use are like this. Photoshop in particular wasn't natively 64-bit until CS4. Prior versions will not use more than 4GB of physical RAM, and that is only if a user knows the registry key to modify that will allow a process to use that much. For the most part, gobs of RAM only helps with multitasking and not with the individual processes past a certain limit. For most apps, that limit is 4GB. For the 64-bit native apps out there, the OS limit is the limit.
2. I'm not a PC gamer anymore, but the only PC games I know of that support 64-bit natively are Crysis and Valve's Orange Box. Simply put, your games will benefit more from a beefier CPU or GPU than from >4GB physical RAM. From the OS perspective, sure more memory is better. From the application perspective, it doesn't know the difference between your 8GB and your 192GB. Virtual memory usage after that point is a whole other issue and best handled by the fastest HDD in one's budget. That will certainly change in the future as devs produce 64-bit native games. For now, 64-bit is not the standard so they can't make that move just yet.
3. Windows 7's max RAM support is different for each 64-bit version. Home Basic is 8GB while Home Premium is 16GB. Versions above that are 192GB, which is plenty. Same story with Vista. Interestingly, none of the versions support the true 64-bit address space maximum. Just useless fact to affirm your geekitude.
4. Virtual machines are the perfect case for more RAM. On a Windows 7 64-bit machine running the appropriate version, the only real limit to the number of VMs a general user can run is the amount of memory their motherboard supports. Even giving the base OS and each VM 8GB of memory space would still allow 23 VMs on the Home Pro or better versions of Vista and 7. Of course for organizations that use VMs, that number might be too low but that's what the Windows Server OSs and server motherboard are for.
Yes, my rant is done...