If you did not already know there is no boot.ini in Vista/LH. So how do you set usermode space to 3GBs?
If you recall on x86 platforms there is 4GBs of addressable VM space. So by default that is split up equally. 2GBs for kernel mode and 2GBs for use mode. Now this is usually fine, but if you have a server that could use more usermode space like domain controllers, SQL servers, exchange sevrers, etc. You could really use another GB for caching of the database in user mode. Or you could suck it up and by an x64 machine.
In 2k3 you would do this by using the /3GB switch in the boot.ini which would split the VM, giving 1GB to kernel and 3GBs to user. Allowing LSASS to cache more of the database on a domain controller for example.
So how do we do this in Vista and LH? It's actually pretty simple:
BCDEDIT /Set IncreaseUserVa 3072
You do not have to specify the boot selection GUID. If you do not specify, it will modify the current selection, which is what you want to do anyways.