I'll add my 2 bobs worth

I have of late changed tact on just how big should my System volume be based ona few factors.

1. How much ram is alocatted (therefore pagefile and memdump needs to be accomodated where ever possible on the System volume) Therefore this relates to which version eg, Standard or Enterprise

2. What apps will this server house and can they all be contained on another volume. "the vast majority of application including IIS can be installed to another volume if possible always try to reserve your system volume for system functions and don't alow log files to write to your system volume is possible"

3. Are you installing R2 SP2, If so add another couple of GB..

There are many other considerations which I'm sure we are all aware of.

What it comes to is :-

Standard Server System volume is 12GB, I think this will very shortly increase to 14GB as I want to ensure I can accomodate for a 4GB page file and have room for a membump if it occures.

Standard Server System volume when running apps which I can't move to another volume eg. SCCM, SQL etc. 18GB Volume

Standard Server System volume when running when running Citrix Metaframe 18GB Volume

Enterprise server System volume when no apps will be in the System Volume 18GB Volume (only accomodate for a 4GB page file on System volume, additional page files are located on other volumes)

Enterprise server System volume when apps will be in the System Volume, they will have 12GB of RAM 24GB Volume (only accomodate for a 8GB page file on System volume, additional page files are located on other volumes)

Then there's the whole x64 question..I'm working on this..