ok this thread is freaking old but I never saw the last replies so I'll reply now because I don't like leaving potential false information in a thread of mine.
there is no need for any 3GB variables on X64 systems
If you read all the documentation you would understand why
a 32 bit system can only allocate up to 2GB to a single process. a 32 bit system with 3GB variables can only allocate up to 3GB to a single process. A 64 bit system can allocate much more than what would be relevant here.
a 32 bit piece of software (such as galciv2) can only get 2GB of memory. a 32bit piece of software with LAA can only get up to 4GB of memory. a 64bit piece of software (which is not the case of galciv2) can access much more than what would be relevant here. (don't wanna search back for the correct values) OF course game Engine limitations apply before that. Which means if a game is programmed to use only 500MB of memory, it could very well crash above it because it would simply not be designed to handle such a large amount.