Is 4GB Ram sufficient for a 64 bit OS

I am aware that 32 bit operating systems cannot make full use of a 4GB installation, however is that extra .5 GB in any way useful when using 64 bit? Doesn't the double word length of memory pointers wipe out the difference? Would 6GB or 8GB be more prudent?

It doesn't matter, 64-bit just allows you to address more memory space. Your memory requirements should be more constrained on what you are planning to do with said system.

You may not even need 64-bit. When it is said that 32-bit can not take full use of 4GB of memory, that is true, but only on a per process basis. Not many processes want that much memory with the exception of database servers.

You Probably Want PAE:
What you may want is an OS that supports Physical Address Extension (PAE). With something like Ubuntu all you need to do is install the 'ubuntu-server' package and you will get the PAE enabled kernel, which will let you use over 4GB, just not for a single process.

Short answer yes, 4GB is sufficient for a 64bit OS.

What OS are you talking about? Or are you talking about all general OS?

It also depends on the application of the OS. More Ram would be suitable for heavy servers etc.. while 4GB is plenty for a desktop.

As you notice, differences are wiped since you have longer pointer types. However, 64-bit with 4 GB enables you to expand at later time. Also notice that address space of process is enlarged so single process can use more memory (limited to atmost 4 GB in 32-bit, usually way less - 2/3 GB for Windows) - whether that memory is physical or supported by page file.

Also, even with less memory, there are applications that require 64-bit - some hypervisiors come to mind as example.

I would go with 64-bit OS provided that driver support is not an issue (yes, all your drivers need to be 64-bit).