Heres my experience with this issue whether totally related to either ASUS M/Bs, or ASUS M/Bs with Nvidia chipsets I'm not sure about that, only the tests I've run relating to my experience is what I'm sharing here.
It seems in what I've discovered so far that the issue is directly related to the memory remapping BIOS feature, when memory remapping is disabled on my M/B in Vista64 I show 2.5G of memory available to Vista, the missing memory is reserved for any components that have onboard memory, in my case I'm running 2 8800GT in SLI each card sporting 512mb of onboard memory, so automatically 1,024mb of system memory is reserved to cover the use of those cards, don't ask me why? I don't know why the M/B reserves this memory for cards that have their own onboard memory, but it does!!!
If I drop to a single 8800GT the memory available to Vista is 3G.
So obviously as with I'd guess most of you, you want the full use of your memory you've paid for, even though you're actually getting it, but you don't realize it, so as for myself I enabled the memory remapping feature in the BIOS setup and Wallaah, Vista64 showed the full 4G available to it, so my thoughts were, wow my performance has to have increased because of this, so I ran benchmarks to test this new developement out, well?
I was disappointed to discover my benchmark performance had seriously!!! Declined???
I challenge you to do your own testing!
Vista64 with 4G available of system memory by enabling BIOS Memory Remapping Feature, 3Dmark06 score = 8,998
Vista64 with 2.5g available of system memory by disabling BIOS Memory Remapping Feature, 3Dmark06 score = 11,744
Thats a 2,746 score difference on the same machine, that seriously affects gaming, which is what 70% of my computer use is all about!
Disabling memory remapping definitely is a performance booster even though in my case Vista can only use the 2.5G.
The full 4g of memory is actually available, just some of it is reserved.
This is my discovery, on my computer, the whys and wherefores, I don't know nor do I care!
So for what its worth I hope this information helps someone! Ryan