Occasionally I need to roll a server that doesn't need 4GB or more of RAM. Is there a downside (performance or otherwise) to just using 64-bit Windows 2008 on everything? This way I'm still in good shape if I need to increase the RAM on the server later.
The downsides are that the less RAM you have, the more you'll page to and from the disk, especially if the server has applications like SQL on it.
It works fine with 2GB. I have servers in 2008 R2 servers in production that have 2GB of RAM in them. They work just fine, but they don't do a whole lot.
Off on a tangent, a relatively idle 2008 R2 VM using Hyper-V dynamic memory sits at around 400MB.
Whilst 64bit memory allocation would take up more memory because of the larger memory tables (not really a lot in the grand scheme of things), you have to remember that a 64bit OS processes will also be able to make of 4GB+ of address space, you are not just limited to the physical RAM, the address space will spill over to the swap space.
The biggest thing in favour is of course the upgrade path. You can always add more RAM to a 64bit OS where as you hit an early wall with a 32bit OS.
It really depends on what processes you are running.