I admit, it does sound a bit wasteful to buy oodles of extra memory, and do nothing with it except hand it over to the disk cache. On the other hand, memory is so cheap now ...
32-bit Windows will begin running into certain limitations at about 2GB, although most users will not notice these. At somewhere between 3 and 4 (usually 3.5 or 3.75), the user will notice unused memory when the reserved graphics memory range is reached.
I would personally put 64-bit Windows on any machine that has at least 2GB of RAM, even though it provides no benefit on the memory front. The reasons, in order of importance:
- Standardization across all machines.
- Leaving open the possibility of future memory upgrades.
- Greater security through hardware DEP and kernel PatchGuard.
At 1GB, I wouldn't bother, but I just got rid of my last 1GB machine.
Windows NT was designed and sold for multi-processor machines from the very first version, in 1993. There are parts of Windows that are serialized, but only a few of these will affect performance on real-world workloads. The most noticeable is GDI+, because people can see the UI slow down. GDI+ rendering now takes place concurrently in Windows 7.